All manufactured parts vary in size, even just by tiny amounts. These small variations can cause problems with how your product fits together and works. This variation or tolerance can be reduced but that increases the cost of manufacture or the quantity or parts that are rejected or reworked.
By carefully analysing the tolerances of the parts and how they fit together, and adjusting the design to be less sensitive to variation, we can increase the permissible variation, reducing cost while simultaneously increasing quality. A win-win for you and your customers!