Codenames Duet keeps the basic elements of Codenames — give one-word clues to try to get someone to identify your agents among those on the table — but now you're working together as a team to find all of your agents. (Why you don't already know who your agents are is a question that Congressional investigators will get on your back about later!)
Collectively, you need to reveal all fifteen agents — without revealing either assassin or too many innocent bystanders — before time runs out in order to win the game. Either player can decide to give a one-word clue to the other player, along with a number. Whoever receives the clue places a finger on a card to identify that agent. If correct, they can attempt to identify another one.
If they identify a bystander, then their guessing time ends. If they identify an assassin, you both lose! Unlike regular Codnenames, they can keep guessing as long as they keep identifying an agent each time; this is useful for going back to previous clues and finding ones they missed earlier.
Don't just take our word for it! Check out this review of Codenames Duet.