What is an "unrevealed key"?

A gift link of some sorts, where you get to press the button that shows the key to anyone for the first time like on many bundle sites. Just guessing tho. :)

Sounds like a scam. You don't know what you get. Probably one of those free games worth nothing.

Humble bundle, indiegala, fanatical all gives you unrevealed key. You have to click on reveal key button to see the actual key. While trading only two things matter gift link or key. There is no 'unrevealed' key in that case.

If they said something like: "This game is new, I have an unrevealed key for it" - then it means something like what Starwhite said.
Likely a key from a bundle site that has never been shown, until they choose to reveal it. Meaning it's an unused key.

If it's from IG, HB or Fanatical, that mean that the key has not been revealed yet. This guarantees the key is not used at all.