Every grinder, to one degree or another, shatters the beans. This fracturing can cause very fine
partials (hence the term, "fines") of coffee grounds that are much smaller than the desired grind level.
The common belief is that fines are bad. That's mostly true. Without SOME fines in a grind the puck would lose a great deal of cohesion because nothing would be filling the space between the grounds that are at the desired grind level. So really, fines are bad when you can tell they're there.
that learned that second point from the folks at Sweet Marias in the below video, but I'm too lazy to watch the whole thing to confirm that it's the source.