This is a defense convention against opponents 1NT opening.
X = 2 suits of the same color (reds or blacks)
2C = 2 suits of the same rank (majors or minors)
2D = 2 suits of the same shape (D and S or C and H)
2M or 3m = natural
Different variations are possible. Note that this method is now illegal under the revised ACBL general convention chart.