What is it, and how does it work?

The Racetrack Playa, seen from space.

  In Death Valley there is a "Playa", a small valley with an endorheic lake (don't ask), it's called Racetrack because of the "Sailing Stones", it's strange but true.
  The valley is about two and a half mile long, the floor is dry compacted sand, and the north end is one and a half inches higher than the southern end (don't know if that makes a difference).
  After it rains, which is infrequently, and it's either a: Cold enough to briefly freeze, or b: starts to evaporate the "sailing stones" go into action.  Not one soul has ever seen them move, or has filmed it, it's been going on for hundreds or thousands of years.  Here's one photo:

A large smooth stone (the track is smooth), you can see the track

The Playa at night, the "arch" is the Milky Way - it's a wide angle lens camera.  You can see one sailing stone,
and the trails of some others.

The  The sailing stones move leaving a smooth track, or rough track depending on the surface of the rock, sometimes rocks will "flip" and leave a different track.  It could be the wind, which reaches speeds of 90 miles and hours, or the freeze/thaw action above; the only constant is the surface is a slick clay when wet, and sand when dry.  Go figure.
Photos by the U S National Park Service.

A rough rock leaves a striated trail.
The rocks only move about every three or four years.