Rifling is the spiraling grooves inside of a gun's barrel. Rifling causes the bullet to spin as it travels down the bore, gyroscopically stabilizing it in flight and increasing the potential accuracy of the weapon. Stabilizing a bullet or projectile is to keep the nose pointed forward. Almost every single modern firearm is rifled, with the exception of some shotguns.
Each projectile has a rifling twist rate that it works best with. Typically, the longer the projectile, the tighter the twist rate has to be to stabilize the bullet in flight.
A twist rate that is too slow will cause a failure to stabilize the projectile. This will cause the projectile to wobble in flight and it may keyhole (tumble into the target).
Overstabilization can be caused by having too fast of a twist rate. In this case, a bullet over a long distance will flip over backwards on itself.
In both cases, failure to stabilize or over stabilization will cause loss of accuracy.
Twist rate is usually expressed as a ratio. The ratio is either in inches or centimeters. A twist rate is said to be one twist (or complete revolution) in thirteen inches (thirty three centimeters). The twist rate of a barrel can be written as 1:13 (33) or 1 in 13 (33).
To find the rule-of-thumb twist rate you can use Greenhill's formula. However, Greenhill derived his equation in ca.1879 based on the conical shaped projectile propelled by black powder. The new way to calculate a twist rate is Miller's stability factor. This factor gives a stability range from 1.3 to 2.0.
See the exterior link for both Greenhill's Formula and Miller's stability factor.