A coffee grinder can never produce 100% of particles of the same size, even if you grind according to your brew method - the finer ones get through the filter. The coarser the filter, the more the quantity of finer grounds which get through. This is called Grind Distribution.
Grind distribution determines the character of final taste in your cup such as the body and bitterness of your brewed cup.
For example, french press has a characteristic body and heavier mouthfeel in its taste, much more than other brew methods. Drip coffee made using paper filters delivers a much clearer taste with a lighter mouthfeel for this reason.
A grind distribution graph measures coffee particle size distribution vs their quantity and looks like a bell curve - a lot of coffee grinds around the average size, some finer than that and some coarser. This is useful in comparing & evaluating commercial coffee grinders. The lower quality the grinder, the more is the spread in particle size distribution. Here is an example:
