1 : a usually diamond-shaped or triangular insert in a seam (as of a sleeve, pocketbook, or shoe upper) to provide expansion or reinforcement
In the case of a sock, the gusset serves to allow extra room from the back of the heel to the front of the foot at the ankle, as measured on the diagonal. It might be a little hard to describe, but it’s an important feature of a sock and can greatly affect the fit. Some folks are just fine making socks without a gusset, but I find those to be too tight at the ankle and usually put in that wee triangle of ease.
And that’s just where I’m at with my Frankensocks; I’m about halfway done making the gusset increases. You can see in this photo that the socks are getting wider, though what you can’t quite discern is that the increase is only happening on the sole stitches:
Only eight more rounds and then I get to do the magical Turning of the Heel. Stay tuned.