In chess, an "only move" can refer to two distinct but related situations on the chessboard. In the strictest definition, an "only move" is a situation in which a player has only one legal move, such as when their king is in check, it therefore must move, and there is only one escape square for it to move to. In this strict definition, the "only move" would be the only legal move, moving the king to the one legal square.
A second, broader definition of "only move" refers to a situation in which there is only one move to maintain a winning advantage, or only one move to avoid incurring a losing disadvantage. In other words, in a given position, the player whose turn it is to move has only one move to maintain the status quo. Sometimes this second type of "only move" can be quite esoteric and difficult to spot, especially if the player is under time pressure.