Board Thread:Questions and Answers/@comment-45.124.201.17-20180112101807/@comment-26447088-20180113022405

They're for the Fantasica crossover Collection event we have going on right now. If you go to the Home screen (where it shows your leader unit), tap "View All" at the bottom left with the events and tap on the "Seize the Winter Seas" icon to access the event.

Collecting the medals gives you event points that progress you towards the rewards and ranking, and you can exchange them for different items.