As the name suggests, referral code comes in use when you refer the game with someone. 

1. Share your Referral Code (found under "Player Profile") to a friend or family member who'd like to play the game.

2. Then, in their game, have them click "Claim" under the Settings icon and enter the code you shared and click Claim.

3. They should then get their 250 bonus gems.

4. And on your end, kindly restart the game and check your in-game mail to claim your 250 gem reward as well.