Question: What does the “X” mean on the card?

Answer: The letter “X” shown to the right of a hand right indicates that the hand is an exposed hand, meaning you may call a tile and expose it to complete a pung, kong, quint, or sextet.  A tile may be called by any player at anytime during the game. If a tile is called the game continues to the right of the player who called the tile. This sometimes causes a player to miss a turn.

Source:   Mah Jongg Made Easy 2024, page 12, CHOOSING A HAND