Used in a variety of applications with nuts and bolts
A washer is a metal ring that sits either at the inside surface of a nut or underneath the bolt/screw head. As the nut tightens to the bolt it presses up against the material it is being fastened to. This allows for a few of things to occur. First, the washer spreads out pressure from the bolt head or nut onto the material making it less likely to cause damage. It also prevents pull-though (a process where the surface area cannot handle the pressure and the fastener slides through the material). Second, the washer can act as a flat surface for the nut to tighten against if the bolt hole was drilled at an angle. Third, the washer can act as a spacer or (in the case of a rubber washer) a seal when appropriate pressure is applied.