The 'X' is used to denote the strength of the stock solution.
Stock solutions are routinely prepared as a concentrated solution, from which one can dilute into a normal working concentration for actual use. Using stock solutions are beneficial because they:
Save time
Conserve materials
Reduce storage space
Improve accuracy
Most applications require a normal working concentration of 1X. For stock solutions with a higher concentration, dilute to 1X before using.
For example: A stock solution that is 500 times more concentration would be written as 500X and would require a 1:500 dilution to restore the normal working concentration. Therefore, to make a 1X working solution you would need to add 2 µL of your stock solution into 1 mL of a buffer of your choice.