With plyometric exercises we intend exercises in which muscles exert maximum force in a very short period of time. They are performed explosively going from a muscle extension to a contraction. This type of exercises are useful to increase power (strength x speed).
Some examples of plyometric exercises are depth jumps, box jumps, plyo push-ups and a lot more. In fact, most body weight-exercises can be executed in an explosive way and thus be plyometric.