A relic of the physical media era. A CD key is a 20-25 character alphanumeric code used to authenticate your copy of the game. On platforms like Origin (now EA App) or Steam, these keys are typically one-time use. The promise of a free CD key for a delisted or hard-to-find game is incredibly powerful clickbait.
This is the engine. The description will have 500+ hashtags (#freegames #cdkey #giveaway #warzone #fortnite #mohairborne). Buried in the middle is a link shortener (e.g., bit.ly or adf.ly).