Electronic rangefinder

I have a Nikon D3100, and it says it has an electronic rangefinder...it sounds nice and spiffy, if I only knew what it was and how to use it! From everything I can gather, it simply tells you how close you are to perfect focus on your subject. However, how would this benefit me (seems a little impractical for moving subjects like people), and how do I use it?

Thanks!

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~Laurie
In love with light!
Nikon D3100, 18-55mm f3.5-5 VR ED kit, 55-200mm f4-5 VR, 50mm f1.8, PSE 9, Lightroom 3
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