Thread is a resource that gives Threadwielders their descriptive name. Technically speaking, there is nothing that prevents any other creature from using Thread as well, but due to the way the evolution of the universe has favoured sophonts tightly bound to the 3-brane's electromagnetic reality, Threadwielders are the only that can intuit its use.
To understand Thread and the effects it can have on the universe, it's helpful to first understand two properties of the setting's universe:
Within this framework, Thread is something that manipulates the patterns within the cellular automaton. It's not remotely unique in this, in that patterns within the cellular automaton generally tend to affect each other. But it is unique in that it does almost nothing else.
Thread is usually be described as having a length, two positions in space-time (its start and its end - the intermediate parts may also have positions, but these are less useful to mention, as on their own they can easily be impossible to pin down for certain for more than a Planck timespan), and information content or payload. (This is true of any line that's travelling through the cellular automaton, even for mundane physical processes such as a travelling photon. That said, not everything in physics is 'line-shaped', whereas Thread certainly is.)
Threads can be chained together (forming a single piece of longer Thread), or their information content can be rewritten by other Thread. A macroscopic piece of Thread in the sense that Threadwielders wield tends to be wrapped in a sheath of non-Thread material (a kind of mirroring substance that perpetuates the Thread pattern within it, preserving it) that allows it to be grasped and moved much like a thing with mass. ('Macro'- or Sheathed Thread as a whole does not have mass, nor inertia. To unskilled observers, it may appear to have something much like inertia in certain circumstances.)
Sheathed Thread is typically inert, in that it's not actively doing anything but hanging around. It's a closed cellular automaton pattern. It takes tampering to get it to do anything else - but when it does 'anything else', it will do it with a ruthless efficiency, propagating as a disruption through the automaton.
Most of the Thread in the universe is used to give it its curvature. This is largely independent of gravity's curvature (which is usually caused by masses, i.e. patterns in the automaton that have far, far, far more complex interactions than Thread) - the macroscopic curvature of the universe as a whole, it's logical geometry. This Thread is called Static Thread and not available for easy manipulation, as it is, for all intents and purposes, infinitely long and unsheathed.
Some pieces of Thread are described only using one position for its start and end. It doesn't make a lick of difference if it's sheathed or not - this Thread has an effective information content of zero. Formally, it still has a position, and it exists, meaning there are more bits than zero, but that's where it ends. It does nothing. It pervades the universe like a fine mist, a perpetual, useless companion, having no function. Sheathed variants can at least be perceived, but are about as interesting as dust particles in the air. This Thread is called Thread Dust.
Neither of these mentioned forms of Thread are interesting to Threadwielders. Instead, Threadwielders generally seek out Free Thread / Untied Thread - Thread with a length greater than zero.
Sheathed Free Thread (usually just called 'Free Thread' - non-Sheathed Thread isn't perceptible, even to Threadwielders, so much of the distinction made in this article isn't relevant to anyone) has the useful feature that it is perceptible to Threadwielders as a straight line of varied length, revealing facets of its information content through the surface of the Sheathing. The Sheathing reacts to geometry changes by degrading in a predictable fashion.
Literally bending Sheathed Free Thread will eventually either:
Threadwielders are adept at chaining Free Thread together using the second feature to encode their desired payload.
Thread payload, once it has left its Sheath, can do three things especially well:
The last two traits are used to adjust curvature, be it gently on a vastly macroscopic scale (as is the case with the Anchor Lines), or on a smaller scale (as might be the case with a Gravity Bomb). The way latter works can be approximatedly imagined in 2D by picturing the Thread growing itself between two points on the Static Thread lattice, then acting as a (temporary) lens for all rules that pass through it.
Unfortunately, the analogy only helps so much - whether viewed from the perspective of 3D or 4D, Thread is effectively a one-dimensional body, whereas a lens would be a two-dimensional body. In reality, the Free Thread and Static Thread exchange information. Ultimately, the Static Thread's very stable information content wins the exchange, but this takes an excrutiating amount of time by any measure of physical processes. By the time reality returns to normal, serious damage has long since been done. That said, for a mortal watching, the 'struggle' is over in less than the blink of an eye.
The first trait's application is more straightforward - anything where alchemy would be useful, whenever you need to create matter out of the vacuum, whenever you want to turn matter into vacuum, whenever you want to turn energy into something else, or really even just whenever you need a big traditional explosion, the ability of Thread to convert is highly useful feature. It can even be used on a very small scale to bio-engineer with high precision.
Sheathed Free Thread is a reasonably plentiful resource in the universe. Unlike Static Thread, it's in constant motion, following wells of gravity. However, unlike Dark or regular matter, it isn't attracted by gravity.
This may seem like a strange assertion - why would something follow gravity wells without being attracted by gravity? The answer lies in a preference for curvature. Thread that's 44 000 kilometres removed from some body's centre gravity will (assuming both no changes in the gravity well such as the body getting additional mass from an impact event, but also no nearby bodies manipulating the gravity well of the body) have a preference for remaining exactly 44 000 kilometres removed from the centre of gravity of that body - no more, no less.
Note that Thread does little to resist being moved from this point of preference. You can picture it like a single corn of dry rice in a perfectly smooth, semi-spherical bowl - if you let it go, it'll slide back to its point of rest at the exact centre of the bowl, but it's trivial to move and hold to any other point on the bowl. That said, unlike the rice corn, Thread moves at an effectively constant speed to return to its point of preference, and as already mentioned, its point of preference does not coincide with gravity's beckon.
Once Thread is unsheathed and delivers its payload, its gone - so it's possible to deplete a region of space of (Free) Thread by harvesting/collecting the Thread and/or using it up. There are no current processes in the universe that would create new Thread (the last Thread creation occurred with the cosmic inflation event). That said, there is so much of it that any travelling Threadwielder is unlikely to run out.
In theory, it's possible to completely undo the universe with Thread. In practise, the necessary payload would be immense and cryptic. It's unlikely even a dedicated Threadwielder would be able to pull it off, even if they wanted to invest a billion years into the endeavour.