Types of concentration units
The “X” factor simply indicates that the solution is in a concentrated form that must
usually be diluted to a “1X” concentration for use. For example, a 5X concentrated solution must
be diluted 5-fold, while a 100X concentrated solution must be diluted 100-fold. The dilutions
are usually done using water.

See more  here under part 3:

http://csmbio.csm.jmu.edu/biology/courses/bio480_580/mblab/SolutionPrepReview.html