I assume you are referring to barrel twist and not some demented government gun demilling process
Twist is the rifling/grooves cut into a barrel during manufacturing process to stabilize the bullet in flight. The barrel twist is basically the amount of spin your firearms barrel will put on the bullet as it travels through the barrel. It is a very important thing related to accuracy of your firearm. The AR15 style rifle can have very different barrel twist depending upon caliber, barrel length and grain of bullet. The twist can actually make the bullet travel faster or slow, but there are limits and consequence to these changes too.
A bullet that is fired with the wrong twist can actually make the bullet do things like fragment in flight before hitting the target. Another little know fact is that a 10" barreled or less AR with about a 1/9 twist, firing a standard 5.56 LEO rd (not military) will generally have less penetration that a 9mm fired from an H&K MP5.
Most standard 16" barreled AR15's you find here in the states will likely have a 1/9 twist, simple because it fires the largest range of bullet weights without overly affecting accuracy when you goto extremes. 55 and 62 grain bullets are the most common you will use at the range and misc shooting.