The X-Card (and its equal and opposite, the O-Card) is a tool for supporting roleplay that involves sensitive subject matter. E.g., topics like slavery, assault, sexual themes, torture, racism, etc. At any time and for any reason, if a player is uncomfortable with what has just occurred, they are to touch the X-Card; this is a non-confrontational but essential signal. "I am not okay in this situation." The GM shows respect and care, moving past whatever scene is at issue in a low-detail/high-abstraction kind of way, so that play can proceed to things that are enjoyable for the participants. As noted, the O-Card is the reverse, something you touch to indicate you are enthusiastic about the current content and would like to see more of it gong forward or would like to engage with it more in the current scene.
This, and other concepts like "lines" and "veils" etc., are tools meant to ensure a certain kind of equality between GM and player. The GM is still responsible for framing scenes (amongst other things, depending on the game in question), but the players are active participants, not simply passive recipients who must either endure whatever they receive or break social norms etc. in order to address concerns they have or unpleasant responses they're experiencing.