Reversi are names for a strategic board game which involves play by two parties on an eight-by-eight square grid with pieces that have two distinct sides. Each of the two sides corresponds to one player, they are referred to here as red and blue.
A move of the red player, for example. Red player must place a red piece on the board, in such a position that there exists at least one straight (horizontal, vertical, or diagonal) line between the new piece and another red piece, with one or more contiguous blue pieces between them. After placing the piece, blue turns over all blue pieces lying on a straight line between the new piece and any anchoring red pieces. All reversed pieces now show the red side, and red can use them in later moves - unless blue has reversed them back in the meantime.
The object of the game is to make your pieces constitute a majority of the pieces on the board at the end of the game, by turning over as many of your opponent's pieces as possible. The player with more pieces on the board at the end wins.