Typically when coffee grows there are two flat seeds (typically called
beans) per fruit. In some rare cases only one rounded seed will form. This
single rounded seed is called a peaberry coffee bean.
The peaberry has a reputation for having a better or more intense flavor profile. This may be the case some of the time but other times the peaberry will be no better than a flat bean from the same crop. One advantage that peaberries do have is that since they are rounded they do roast a little more evenly and a little easier.
This is especially important in the case of rustic roasting methods where agitation may not be as thorough as in more modern roasters. This is one reason that it is believe that peaberries have a traditional reputation for superior quality.
Some popular regions that separate and export peaberry coffee beans include Hawaiian, Tanzania and Kenya. The process is labor intensive and typically only makes sense for countries where the coffee has a good reputation to begin with, and therefore the demand for a premium or special grade of coffee bean justifies the investment in picking them out and selling them separately.
Ultimately, peaberries aren’t much different from their smaller siblings – the flavors will be very similar (coming from the same trees). The primary difference is to the roaster, who may have an easier time developing a consistent roast from a uniformly sized coffee bean.