shape the gusset

This is a gusset. While is has one of the weirdest names in knitting, the gusset isn’t all that weird. Gussets actually appear in other parts of clothing to help make them fit over the many curves of the human body. In socks, the shape being addressed is just the part of the foot that gradually narrows from where your ankle meets your foot to the size it should be to fit your arch.