[VIDEO]http://www.youtube.com/watch?v=KTfnp5W4peg]Drum Editing in Reaper - YouTube[/VIDEO]
Thread on the Reaper forums explaining it.
It's purpose is to intelligently fill the gaps between items that have been cut up and repositioned in the timeline, usually for drum editing purposes (ie. cutting all the hits and snapping them to the grid.) It does this by trimming the starts of items left to fill gaps, time stretching items if the gap is too big, etc. all according to preferences laid out by the user in the initial dialog.