An O-ring (also known as a packing or a toric joint) is a mechanical gasket in the shape of a torus. It is a loop made of elastomer with a circular cross-section. It is designed to be placed in a groove and then compressed during assembly between several parts. This seals the interface.
To block any path that could otherwise allow liquids or gases to escape , O rings are used. To secure the o rings, place them into a groove and then compress between two surfaces.