Remember the CDs? First the CD, then 2X CD, the 4X CD, etc. Same deal here. It is the data transfer rate. If the sd card has 50X on the label, it theoretically is capable of 50 times the data transfer rate of the original.

If you have a regular sd and take a snapshot, it takes lots more time to render the snapshot than say a 100x sd. You can actually see the difference on the screen between the 2 speeds when snap shooting.

speed is written on sd (like 100X). Same as old CD....