When using an AeroPress, there is such a thing as a "just right" grind — not too fine and not too course.
If your coffee grounds are too fine, the immersion process will over-extract the grounds, resulting in unpleasant bitterness.
If the grind is too coarse, the grounds are not exposed enough to the water, and your final cup loses out on all its potential flavor. Under-extracted coffee tastes sour and unpleasantly acidic.
The ideal AeroPress grind setting is a medium-fine grind, resembling the texture of table salt. This is best achieved with a burr grinder.