Twist imparts spin on a bullet which dramatically improves its stability in flight. For example, if a barrel has a twist rate of 1 in 9”, then the rifling would complete 1 full revolution in 9 inches.
Heavier bullet weights in a given caliber require more twist. More twist is indicated by a lower number- 1 in 9 has more twist than 1 in 12.
Firearm manufacturers select twist rates for their guns which effectively stabilize most popular bullet weights for that caliber. If bullets are used in a given firearm that are on the extreme ends bullet weight for its caliber accuracy of the firearm may be effected.