Cavity Back
An iron design with a hollowed-out back that moves weight to the perimeter for added forgiveness - the most common iron style today.
A cavity-back iron has, as the name says, a cavity carved out of the back of the head. Removing mass from the center and redistributing it around the perimeter raises MOI and makes the club more forgiving on off-center hits. It is the default design for the vast majority of irons sold today.
Cavity backs sit in the middle of the iron spectrum, between the compact blades favored by skilled players and the oversized max game-improvement irons built for maximum help. Within the cavity-back category there is wide range - from sleeker "players cavity" designs to chunkier game-improvement builds.
The benefit is straightforward: more consistent distance and a more playable miss when you do not catch the center of the face. The cost, relative to a blade, is a little less feedback and slightly less ability to intentionally shape shots, though modern cavity backs have narrowed that gap considerably.
For most golfers from beginner through single-digit handicap, a cavity back of some kind is the right call. The question is usually how much forgiveness to add, not whether to have any.