One-man means one person does the whole thing: they perform it, run it, or control it alone. A one-man show is a play or concert with only one performer. A one-man business has no staff. A one-man band, in the old picture, is a musician who plays many instruments at once; people also use it for someone who tries to do every job in a team. It usually comes before a noun. Though actually, the man here is old-fashioned English for “person.” You will still hear one-man in job talk and reviews, but many people now say one-person, or one-woman when they want to be exact. The meaning does not change: still only one human being. It can sound admiring (they did it all themselves) or a little worried (no one else is there to help).