A polycap is a small sleeve of soft polyethylene seated inside a joint to grip the peg that plugs into it. That slight rubbery give is what lets a snap-fit model hold a pose after hundreds of movements without wearing loose.
Polycaps ship on their own runner, usually molded in dull gray. Two care notes: paint does not stick to them, and solvent cement does not bond them — they are handled dry.
See it in context in our kits & grades guides, or explore the tools hands-on in The Bench.