Barrel twist, or rifling twist, defines how many twists of rifling occur within a certain length of barrel. For example, a barrel that is 1 in 14" means that the spiral rifling makes one full twist of the bore surface in 14 inches.
Knowing rifling twist is important in choosing your bullet weight. As a rule of thumb, the heavier the bullet, the tighter the twist should be to stabilize the bullet in flight. In my M1A, I shoot 168 gr. bullets and find that a 1 in 10" twist barrel is ideal. 1 in 14" would not provide the same stability, but may be just fine with a 150 gr. bullet.
Hope this helps.