My Nintendo is a rewards program in which coins can be earned and exchanged for games, dlc, discounts, and more.
There are three types of coins that can be earned on My Nintendo:
Miitomo coins are earned by completing various actions in Nintendo's smart phone app Miitomo. These coins count as platinum coins when redeeming them for rewards
Platinum coins are earned by completing various missions which can be found on the My Nintendo website. Some of these missions include linking a social media account, signing into the Miiverse weekly, or signing into the e-Shop weekly.
The gold coins are earned solely by purchasing items on the e-Shop. There are various tiers of gold rewards that are given depending on how much money is spent in the e-Shop.