An iron design with the back of the head hollowed out so the removed mass can be pushed to the edges of the face. That's perimeter weighting in action: the club resists twisting on off-center hits, so a strike toward the toe still flies reasonably straight and loses less distance. Cavity backs are the default recommendation for most golfers — roughly everyone who doesn't hit the center of the face nearly every time. The trade-off is a slightly chunkier look and a bit less feedback and shot-shaping feel than a blade.