This is a round coffee seed (bean) that was formed because of incomplete pollination of the flower, thus producing only one seed inside the cherry. Two seeds (not attached as one like a peanut) is the normal cherry content.
Having nothing to press against, the seed grows round instead of having one flat side. Many say that this concentrates the flavor of two beans into one. My experiences does not bear that out, although I will say that artisan roasters do notice that peaberries roast more uniformly. Maybe that produces better flavors than their flat sisters.
Peaberries grow on the same trees as do the flat sided beans. They are rarely more than 5-10% of a tree’s production. They are separated out during the grading process by air separators or size-grading equipment.
Are they worth the extra price? I think not. Form your own opinion when you get the chance.