Back Up Rings are washer-like devices that are used to prevent a seal from extruding through any gaps while under pressure.
These seals are installed in the downstream side of the gland.
Back Up Rings are not seals themselves, but are usually used in conjunction with O-Rings in high-pressure applications for support or as a spacers.
Typical applications include hydraulic cylinders and high-pressure hydraulic systems, and high-pressure valves.

Back-up Ring Material