Compression fitting olives are used in electrical conduit systems and plumbing. They are generally used for fastening, joining and sealing or for reinforcement. They're the safe way to connect copper pipe without the use of heat.
Compression fitting olives work by the compression of an 'olive' between a valve and nut, and the pipe itself. The valve and nut both have tapered sides into which the olive fits. Using a spanner and a pair of grips you can tighten the nut to exert pressure on the olive. The compression fitting can be assembled by sliding on the nut and olive at both ends, and tightening the nut with a spanner. In the case of a leak it's advised not to over tighten a compression fitting so that the olive remains undistorted.
Compression fitting olives are most often narrow circular rings made from metal, and less commonly, plastic. They join two tubes or pipes together and are compatible with pipes of a variety of standard sizes.