SpoilerI figure it's a special thing. The third or town/city you reach after you started off, perhaps the harbor town on the second island, will have a festival going on. There you can play some sort of (mini)games, preferably something with your pokémon, like the pokéatlon, to win these coins to buy outfits. Later places will hand them out as well. On the third island you get fly, so you always have a quick way back there.
Yes, that is way to specific for such a wild guess. Please just ignore everything after the first sentence.