In chess, a "known move" is a move that has been played even once before in the recorded history of chess in that exact position on the chessboard. A known move is contrasted to a "novelty," which is a move that has never been played before in that exact position in the entire history of chess.
Because chess is such an old game, and has been around for thousands of years, at high levels of play, top players often play dozens of moves at the outset of the game that have been played before and intensively studied in chess theory. These are the known moves, and have well-known responses. It is not until one of the players finally plays a novelty that the game really begins.