What's $0 down payment or Pay today with tmobile?

I'm looking for a new phone for a new school year, but I don't think I can afford a $700 phone. I don't think I have "Jump!" But when I go to my account and look for an upgrade it says $0 Pay today! Does that mean its free? Or do i have to pay over time? I just want an Iphone or a Galaxy s4… Is there any other phone thats good if i can't get this one?

It means you don't have to pay the 700 today

Payments of 27 dollars a month will be added to bill to pay for the phone