Taciturn describes a person who says very little — not because they are shy or busy, but because that is simply how they are. It is a settled quality of someone's character, not a mood on one particular day.

The part learners most need is the flavour. Taciturn is not a compliment and not quite an insult; it sits in between and leans slightly cool. A taciturn person can come across as unfriendly, hard to read, or unwelcoming, even when they mean nothing of the kind. That edge is built into the word, so choose it carefully when describing a real person in front of you. If you want to praise someone for not talking too much, English gives you other words: quiet, reserved, a man of few words, softly spoken.

It is quite literary. It appears in novels and in careful journalism — the taciturn farmer, the taciturn detective, the taciturn coach who gives one-word answers to reporters. In real conversation it is uncommon, and using it in a casual chat will sound like you have swallowed a book. Notice also that it tends to be used of men more often than women in older writing, which tells you something about the world those books came from rather than about the word itself.

The matching noun is taciturnity, and the opposite direction is talkative, chatty, or outgoing.