A gusset is a patch of material added to a garment to reinforce it at a point of maximum stress.
In a fetish context, it generally refers to a double thickness area in a pair of tights covering the crotch. There will be strong seams all around the gusset. Viewing this area will often be the aim of someone trying to get an upskirt look. This area will be missing and replaced by a large hole in crotchless tights.
Tights that are "sheer to waist" will not have a gusset.