Tresses means a woman's long hair — but not neutral, plain hair. The word carries admiration built into it. If you call someone's hair her tresses, you are saying it is long, thick and beautiful. You cannot use it for short hair, thin hair, or a man's hair in ordinary writing.

The word belongs to poems, romance novels, fairy tales, and advertisements for shampoo. Nobody says it about a friend at lunch. If a modern speaker uses it in conversation, it is almost always playful or teasing — 'time to get my tresses cut'. Used with a straight face in normal speech, it sounds old-fashioned and a bit sugary, and a listener might find it strange.

It is normally plural: her long tresses, golden tresses. The singular 'a tress' does exist, but it means one lock or braid of hair — the piece you might cut off and keep — and even that is rare and old today. So for students: learn it as a plural word for beautiful long hair, and expect to meet it in stories, not in speech.