The X rating is a measurement of the performance of the card. It is determined by dividing the write speed of the card by 150KB/s*. To put it another way, you can determine the write speed of a card by multiplying its X rating by 150KB/s. For example:
50X = 50(multiplier) x 150KB/sec = 7.5 MB/sec
* 1 KB = 1,000 Bytes; 1 MB = 1 million Bytes.