Let's start at the beginning: An MPG meter is a device that tells you how many miles per gallon your car is getting (or how many kilometers per liter, for those on the metric system). Some vehicles have this feature on their onboard computer with varying degrees of sophistication
That's where aftermarket MPG meters come in. There are many models out there, and once they are installed, they will tell you how fuel efficient your driving is. But it's not only "oh, that's interesting, now I know." Rather, if used properly, they become an active feedback mechanism that can help you save gas, save money, and pollute less.