League of Legends Gift Card can be redeem for Riot Point (RP) which is one of the primary currencies used to unlock League of Legends content, the other being Blue Essence. Riot Points are purchased with real-world currency, and are only used in the Riot Store.

1. Launch the League of Legends client or go to signup.na.leagueoflegends.com to create an account. 2. Click on the Store button. 3. Click on the Purchase RP button. 4. Select Prepaid Cards & Riot Pin and enter your code. 5. Click Submit.

https://www.riotgames.com/en/terms-of-service