You can go ahead to click on the gift card link included in the email sent to you. Once that is done, the 1-year free Premium will be applied on your next renewal. You will then be bill $0 instead of your actual next renewal plan amount. However, please do ensure to do so by the expiry date stated in the email sent to you.
The next payment information is also shown in the Membership Details page under the App's Profile Settings (Profile>Settings>Burpple Beyond Membership Details).
*Please note that the 1 free year of Premium as a referral reward is capped at ONE per Burpple member, as stated in the Referral Program's T&Cs.