It is normally used to distribute the load of a threaded fastener, such as a screw or nut. Plastic washer is a thin plate (typically disk-shaped) with a hole (typically in the middle). Washers usually have an outer diameter (OD) about twice the width of their inner diameter (ID).
Washers are usually plastic. While they may look similar, washers and gaskets are usually designed for different functions and made differently.
High quality bolted joints require hardened plastic washers to prevent the loss of pre-load due to Brinelling after the torque is applied.