Simply Trophy, when the character Marie get's introduced in the velvet room you will have a few more options on hand. One of them will be to register and buy skill cards. Skill cards are the red cards with a number 1-2-3 ect with a picture of a sword you obtain when you get shuffle time after a battle. They are needed to teach any Persona any skill, you will naturally came across many of these will rescuing people and exploring dungeons in the game.
All you have to do is register the Skill card with Marie from the menu in the Velvet room and the Trophy will pop. Remember when you register a skill card you actually lose it entirely, but you are able to purchase it whenever you please from the list of available ones so you can use them whenever you want to.
Note it would be in your best interest to try and obtain as many as you can through Shuffle time as you need 100 skill cards across the game for another Trophy.