Referral codes are meant to bring in new people, someone can use your referral code to create their account, and if they deposit $200, you will both receive $20* in BTC.
*$20 in BTC is distributed when a new user makes the first deposit of $200 or more using your referral code at the time of signup".
You can find your referral code by tapping on the "Profile" button and then on the "Refer your friends".
Please note that referral codes cannot be applied to previously created accounts, the code has to be entered before the account is created. Press on "Have a referral code?" underneath the Create account button to enter your referral code.
Once you have entered the correct referral code you will see a confirmation screen and you can continue making your account.
**The referral reward will be locked for 30 days after which it will be unlocked. If the funds are withdrawn from the account during the 30 days lock period the reward will be canceled and there is no way to receive it again.