3DS Menu Golden Coins?

What do they do or mean. it says i have 31. I don't know how i get them or what they are for.

They are used in some games to get items etc or to unlock things in SwapNote/Letter Box, you get them by walking around with your 3DS in sleep mode.

You get 1 coin for every 100 steps taken and can get 10 coins a day and have a maximum of 300 at any one time.

There are plenty of games that use them. Kid Icarus uses them to buy more eggs. You can spend them for additional clouds in Pokemon Dream radar. You can also use them for more warriors in mii plaza and more puzzle peices. Can't believe you don't even know what they are.... I need more.YoshiYogurt

They can also be used in AR Games to buy more games as well as for trophies in SSF43D.

The game I used them the most in, other than the built-in games, is RE: Revelations. You could use them to buy weapons for Raid Mode and from time to time you can get something really good.

You'll also be able to use them to buy Fortune cookies in Animal Crossing: New Leaf which can be redeemed for items.