The Fit Meter is a small circular plastic peripheral that comes packed with the Wii Fit U and has a pedometer, altimeter and an atmospheric sensor in it, which you can take with you on your daily exercise / routine.
For example, the pedometer tracks the number of steps you take when you walk, jog or run.
The altimeter determines at what level the number of steps you took. So for example, if your daily walk includes going up and down steep roads or paths, you will get more credit than compared to a walk along a straight road.
The atmospheric pressure sensor will measure your distance from sea level i.e. how high above or how low you are compared to sea level. This is because it takes more effort to walk or jog if you are in an area above or below sea level since the air is thinner there.
You can then point the Fit Meter towards the infrared sensor on the Wii U Gamepad and the Wii Fit U game will compile a complete visual report of your overall activity and tell you how many calories you have burned.