A rubber washer is a ring made of rubber

Rubber

Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...

 used in mechanical devices. It is used to prevent vibration from spreading from one part to another, reducing the noise levels.

Typical uses are mounting computer parts, like fan

Fan (mechanical)

A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...

s and hard disk drives. By decoupling the motor from the computer case

Computer case

A computer case is the enclosure that contains most of the components of a computer...

 it prevents the resonance chamber

Resonance chamber

A resonance chamber uses resonance to amplify sound. The chamber has interior surfaces which reflect an acoustic wave. When a wave enters the chamber, it bounces back and forth within the chamber with low loss...

effect from amplifying the noise.