Not praised, not thanked, not famous — even though the person (or thing) did something that deserved praise. It is almost always used before a noun, and by far the most common phrase is "unsung hero": someone who does important work that nobody notices or says thank you for. The cleaner who keeps a hospital safe, the mother who works two jobs, the assistant who does the real work while the boss gets the award — these are unsung heroes. You can also say "unsung heroine", "the unsung heroes of the war", or stretch it to things: "garlic is the unsung hero of this dish", which is a light, half-joking use you now hear a lot in food writing. The word carries a small feeling of injustice — the speaker thinks the praise SHOULD have come and did not. So it is warm toward the person and quietly critical of everyone else. It comes from an old picture: in the past, heroes were praised in songs and poems, so a hero nobody sang about was forgotten. That picture is dead in the modern word — nobody is thinking about singing — but it explains the shape. Note it is not a synonym for "unknown" or "unpopular": an unsung person may be well known, they just are not given the credit. And it is not used after "be" in normal speech; "he is unsung" sounds odd, "he is an unsung hero" is natural.