This is a really cool card trick that involves two people working together. There is a code between the two of them that communicates what the key card (the one they have to guess) is.
There is a little bit of math involved but it just takes a little bit of practice between the two people to be able to pull this off.
This trick was developed by a mathematician named Fitch Cheney. If you want to see the written version that explains the trick you can see this blog post HERE You will see the whole explanation about the coding and how the first magician communicates with the second one to let them know what the card is.