Phone topic! AT&T Early Termination Fee

So , i don't really understand this one .

I'm purchasing a phone http://www.amazon.com/Nokia-Lumia-Windows-Phone-Black/dp/B007P5NHJO/ref=sr_1_1?ie=UTF8&qid=1336025899&sr=8-1 .

But it adds some kinda early termination fee . What does it really stands for?

If you return it within XX days you will have to pay the penalty?
My english is not that perfect to understand that stuff >.>

Anyone?

Thanks!