Paired intrinsically with StreetPass, the 3DS Pedometer is the second part of Nintendo's efforts to a) get you to carry your 3DS with you wherever you go, and b) help you to keep fit and active.
Found in the Activity Log (which is on the main menu), the Pedometer is, at first glance, a fairly standard way of measuring your steps. Giving you a step by step (do you see what we did there) breakdown of your walking throughout the day, the top of the screen is split into an hourly breakdown of your movements throughout the day, while the total number of steps you've made both that day, and since the program began counting is displayed above.
Somewhat disappointingly, it's not possible to set your step size, so it's never going to be that accurate - and, due to the size of the 3DS, it's somewhat hard to fit it somewhere where it's only going to bounce when you move - like on a belt buckle, like a normal pedometer. It's intended to be more of a guide to how active you are in the day than the be all and end all for calculating how far you've walked (there's no break down in miles here, either), but the 3DS pedometer does have some interesting functions.
Every 100 steps you take, you'll unlock a Game Coin - the total of which are shown at the top of the screen. For every hundred steps you take thereafter, up to a total of a thousand steps, you'll unlock another Game Coin, giving you the ability to unlock up to ten a day. These coins can then be spent in various games and applications. Whether you're buying new heroes in StreetPass Quest, or new characters in LEGO Star Wars 3, it's an interesting way of rewarding you for efforts - and the steps you take.