A rather obscure feature of the MIDI standard. When a key is released, the keyboard can measure how fast the performer allows the key to spring back up, and insert the value into the resulting note off message. What the receiving entity is supposed to do with this information is not specified in the MIDI standard. In practice, few synths or keyboards implement release velocity. Of the few that do, the usual use is that on a monophonic synth, or in a patch that has been set to a monophonic mode (which only allows one note to be played at a time), if two keys are held down and then the key that has note priority is released, the release velocity is used as the velocity value for retriggering the note that is still held.
EDIT: Release velocity is used for modulating the volume of the sound a piano makes when relasing a key. That way a fast release of the key causes a louder playback of the release sample that a has been recorded.