These are the Syzygy 3-4-5 chess endgame tablebases. This is a database containing every chess position with 3, 4, or 5 pieces, and whether each position is a win, draw, or loss (and the number of moves to win or lose given perfect play), as well as the optimal move(s) in each position.
See https://en.wikipedia.org/wiki/Endgame_tablebase or https://chessprogramming.wikispaces.com/Syzygy+Bases for more information.