>>13026426The 3DS has a pedometer in it and for every 100 steps you take (with a cap of 1000 steps, and thus 10 coins), you get one Play Coin.
They build up over time basically depending on how active you are (and whether you bring your 3DS with you everywhere or not) and you can use them for various things. Some games let you use them as currency. They serve a function in the stock Mii Plaza games. They can and probably will gain new purposes in the future.
