Your friend(B) tells a third person(C) to pick any 5 cards out of the deck of 52 cards without any jokers. That person can inspect, shuffle your cards i.e. the cards are genuine and have no markings.
Now starts the fun part. He(C) picked the 5 cards and handed over to your friend(B). B looked at the cards and picked 1 card out of them and returned back to C without showing it to anyone else. Afterwards B rearranged the remaining four cards in a special way, and gave those cards to you.
You looked at the card and told C what card was returned back to C
You have to devise such a strategy that is known to B (your friend) and that special way that will eventually tell you correctly, what was the card returned to C (the third person)