Inside the fruit, you see the white pith on the underside of the peel, as you do with other citrus fruit. The flesh of the fruit is greenish, and not very juicy (in fact, you could basically say “juice free”, if you wanted to spin that into a marketable attribute.) So thick is the rind, though, there’s in fact not much flesh at all, and what there is, is very, very sour.
The fruit, though, are very fragrant; the scent of just a few in a bowl can fill a whole room. Aside from that, the fruit is really only used for its peel, which is candied. To make the candied peel, the peel is first soaked in brine to cure it, then candied by boiling in a sugar syrup.
Citron varieties include Etrog Citron and Buddha’s Hands Citron.
Citron was the first citrus fruit. It was probably introduced to Middle East between 400 and 600 BC (there are the usual Alexander the Great stories, of course).
Romans mostly grew it as an ornamental garden tree, and for some medicinal purpose, but they didn’t really draw on it for sour notes in their food.
The greatest thing that Citron has done for us, however, is that it is the ancestor from which all other citrus fruit sprung.