The shape is an X – slanted lines crossing at the middle of each line.
There are two major representations of Cancel/Stop icons - a square and an X. The square was used on the old tape players and usage is common in current device media players. The X shape was selected for a number of reasons. The X has more tactile features, more edges, that facilitate quicker tactile recognition. The thought is a stop or cancel button is probably used in a time constrained situation so the easier and quicker to locate, the better. Also the square is fairly generic and may be applied for other functions, such as an unlabeled button. The X shape is more unique.