One of the first things you are introduced to in the game is the Stamp Card, which is given to you as incentive to basically do the sidequests. The majority of the sidequests and some of the main quests will reward you with a Stamp upon completing them. There are different Stamp Cards, with Platinum being the third one you get. In order to fill it up, you will need to collect 50 Stamps, in addition to the 30 to fill up the Silver card and 50 to fill up the Gold one.