Grind consistency is the measure of uniformity in the distribution of particle size in a batch of ground coffee.
Having a high peak and a narrow distribution is usually seen as being desirable. This is because it ensures that most of the particles extract at the same rate.
Fines (the dusty, super tiny particles) are usually considered undesirable because they tend to over-extract and create bitterness. They also clog up filters or screens when making pour over coffee, which causes stalling and clogging.
If a grinder has two slightly separated peaks, one bigger and one smaller, we call it a bimodal grinder when we look at the distribution. If there is only one peak, we call it unimodal.
Unimodal is seen as the holy grail for filter coffee, whereas bimodal is regarded as more suitable for espresso.