The Five Card Charlie sign was a decoration in Chandler and Joey's apartment. It reads "Five Card Charlie pays 5 to 1", possibly referring to a rule in Blackjack.
It first appeared in the season 4 episode The One With All The Wedding Dresses, on the wall between the front door and the fridge. It moved to the wall above the sink in The One Where Ross Hugs Rachel, and then to the wall on the other side of the front door in The One After Joey And Rachel Kiss.