The literal meaning of the verb unwind is exactly what it sounds like — the opposite of "wind," or twist. But when someone says they need to unwind, they mean that they need to relax.
When you come inside on a cold, snowy day, you might first unwind the scarf from around your neck, loosening and untwisting it. If you then grab a cup of hot chocolate and put your feet up on the coffee table, you’ll feel yourself start to unwind, or become less tense. Unwind has been in use since the fourteenth century, but the figurative meaning of “to relax” dates only from the mid-twentieth century.