A shoulder rest is something which is used by people who play the violin or the viola. It may be made of wood, aluminium, carbon fiber or plastic. Usually, the shoulder rest attaches to the edge of the violin with "feet" padded with rubber tubing or made of soft plastic.
The purpose of a shoulder rest is to help the player to be comfortable holding the violin, which is held between the chin and the shoulder. Not all players use a shoulder rest. Many violinists prefer just to put a piece of cloth or some other form of padding on their shoulder. Whether a player uses a shoulder rest may depend on whether he or she has a long neck, and whether he is wearing clothes made with thick or thin material. It is important that the player can shift his left hand easily and freely to play in the higher positions without being afraid that he will drop the instrument, and without feeling the need to squeeze the instrument between his chin and shoulder as this will make the muscles in his neck very tight.
The shoulder rest must not be confused with the chinrest, which is the bit on which the player rests his chin.