Purchasing this grants you access to the premium track of the battle pass inside the gui menu. Open your gui and navigate to user dashboard then to battlepass to view the possible items you can get upon purchasing this. The premium track purchase only works for the current season.

Purchasing this item after having claimed battle pass rewards items will allow you to catch up to your current spot in the battle pass, so you can purchase this premium track after completing the free track and get all the items at once.