Resolver Scavenger is a gear perk that gives you extra crafting materials when you loot. Materials pay for weapon mods and Resolver builds, so it speeds up your upgrades over a long play session.
Pickups that give crafting materials give a bit more. Over hours of looting that adds up to more mods and attachments at the workbench.
Wear the Scrounger Coat in the chest slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Good early, when materials are tight. Later you will usually swap the chest slot for a defense or combat perk.