An item activates the effect bonuses for the collected medals function, as the collected medals help increase your base attributes.
You can get trophies from the Black Market deals and some events.

You can use it to enhance the effects bonuses of activating a group of medals by clicking on the collected medals, then clicking on the star on the right of each group to activate the bonuses after you ensure the activation requirements are fulfilled.

Note:
When you get the golden trophy, it will not be added to the inventory; instead, it will be inside the combining effect bonuses window for the collected medals.