I honestly don't know how to describe this Unity add-on. I call them thighs from "adult anime", if you get my meaning, but the creator of this tool, nHaruka, says it makes indentations where clothes and body meet to shape an effect of fabric digging into the skin.
nHaruka creates assets and physics for VRChat, so I assume this add-on, which allows you to adjust various parameters, like smoothness, will also appear on the artist's Booth once it undergoes testing.