(insert screenshot of someone that had gained one referral code)
Referral Codes are functions in the game My Singing Monsters that give free 5 free diamonds to both you and another player, allowed only once per account. It's intended to be a special thank-you for the person who introduces you to the game, though it can be used for someone else.
You might see messages in various places which promise a much larger number of diamonds if you enter some code for the referral bonus. These messages are lies, trying to trick you into using up your bonus for the benefit of the person who posted the message.