A liner is the inside layer of something that adds protection, comfort, or an easy-to-clean surface.

A trash can liner is the bag you put inside the can so the can itself stays clean; when it’s full, you tie it up and toss it. A jacket liner is a softer, often warmer layer sewn inside a coat, so the coat feels nicer against your skin and holds in heat. A curtain liner can block light and help keep a room warmer. A drawer liner is a thin sheet that protects the wood from scratches and spills. Even a pool liner is a big waterproof layer that holds the water in.