A recherché subject, idea, word, or taste is uncommon, and someone has chosen it on purpose. Often the quiet point is to look clever or refined — see how much I know, see how fine my taste is. English borrowed the word from French as it is, so it already feels fancy; you meet it in reviews, academic writing, and prose that itself wants to sound cultured. It is not always unkind, but it often comes with a raised eyebrow: this was picked to impress. Though actually, it can also mean only ‘very carefully sought out,’ without the showing-off tone — the rest of the sentence tells you which. It is a high, rather rare word. Most people will just say rare, obscure, or fancy.