In most cases it just means “not in a relationship” and doesn’t tell you about marital status. But if you’re in specifically asking about marriage, it can mean unwed (and I’m sure it gets even trickier for poly relationships).

If a flirty stranger approaches and asks “are you single”, you’d say no “no, I’m in a relationship”

If you’re filling out a form at the doctor office and it asks “marital status”, you’d put “single”.