It is the sound produced by hitting the rim and the head of a drum at once, with a drum stick. Rimshots are usually played to produce a more accented note, and are typically played loudly. However, soft rim shots are possible.
There are two standard types of rim shots in marching percussion. The first, more common shot is called a "ping shot". In a ping shot, a drum stick hits the head and the rim at the same time, with the bead very close to the rim. This produces a high pitched sound. The other, called a "gock," is produced by putting the bead of the drum stick close to the center, the rim making contact closer to the hand than in a ping. This makes a lower sound.
http://en.wikipedia.org/wiki/Rimshot