It replaces the default meshes or textures for the underwear and swimwear that appear in Create-A-Sim (CAS) and when a Sim is in their "undressed" state.
If the mod adds transparency or removed censorship, you may also need a "No Mosaic" or "Censor" mod, such as the one from SimsVIP . Folder: Adult World TS4 - Sim File Share
The item you're referring to is a for The Sims 4 by the creator Noir and Dark .