A Skid Plate is a piece of plastic that attaches to the underside of the tail and/or nose. It is designed to prolong the life of a deck by protecting the nose and tail from wear tear caused by skidding. This is more important for a freestyler than a street skater as many common freestyle maneuvers wear the nose and tail down very quickly and this would get very expensive as deck replacements would be frequent.
Skid Plates are also known as Skids, Tailsavers, Grind Plates, or Tail Bones (Powell Peralta).
Here is a great video by Tony Gale's on "How to install (and why you should use) skid plates"