Your prepaid code can be redeemed for RP, which can be used to purchase in-game content for League of Legends.
To redeem your code in the game, log in to the League of Legends client and follow these steps:
Start by clicking on the “Store” button:
Next, click on the “Buy RP” button:
Select the “Cards and Prepaid Codes” option:
Finally, enter the card code and click on “Submit”:
You can now enjoy your RP points:
Video Explanation: