Rifling
Also known as: lands and grooves
The spiral grooves cut or formed into a barrel's bore that spin the bullet for stability — the raised areas are 'lands', the recessed are 'grooves'.
Rifling is the set of helical grooves inside a barrel that grip the bullet and impart spin as it travels down the bore. The raised ridges between the grooves are called lands; they engrave into the bullet and seal the bore so propellant gases push efficiently. Rifling can be cut, button-pulled, hammer-forged, or polygonal in form.
Spinning the bullet stabilizes it the same way a thrown football spirals, dramatically improving accuracy over a smoothbore. The rate of spin is the twist rate. Shotguns are typically smoothbore (hence 'gauge' and shot loads), while 'rifled' slug barrels add rifling to stabilize a single projectile.