Tarragon is a herb — the thin green leaves of a small plant, used to give food a particular taste. The leaves are long and narrow, and the flavour is sweet and slightly like liquorice or aniseed, with a faint pepper edge. It is one of the central herbs of French cooking: it goes into béarnaise sauce, into chicken dishes, into vinegar, into mustard. Because it is so tied to French food, English speakers often meet the word on a menu before they ever meet the plant. You can use it fresh or dried, but dried tarragon loses a lot, so cooks who care buy it fresh. Note that there are two kinds sold: French tarragon, which has the real flavour and is what recipes mean, and Russian tarragon, which grows more easily but tastes of almost nothing — a small trap for gardeners. The word is uncountable in cooking talk: you add tarragon, not 'a tarragon'. It is a fairly high-register food word; a person who says 'tarragon' in English is usually someone who cooks, reads recipes, or eats out.