"Kingside" and "Queenside" (or "King Side and Queen Side", I can find examples of both usages online) are chess terms for the side of the board closest to the king (files e-h) and the queen (files a-d). For the player with the white pieces, kingside is right and queenside is left, while for the player with the black pieces, kingside is left and queenside is right. While this might sound like a confusing affectation, since the terms "right" and "left" would seem to work just as well, the reason for it becomes clearer (for example) in discussions of whether to castle to the kingside or queenside, rather than to castle "to the left or right." Like many things in chess, the terminology around "Kingside and Queenside", while confusing at first, makes the game easier to understand.
Side Quest 2024