How does t-mobiles new upgrade system work?

I haven't upgraded my phone since t mobile has did away with the old pay one price for an upgrade for two years. I'm planning on pre ordering the 32 gb green iphone 5c but I noticed there's a thing that says something like, +$21 x 24mo. Or pay full at check out $549.99. I'm confused. Someone help me out please? I don't want to pay per month or 500 at checkout.

Well its not free so you have to pay somewhere. You can get contracts with others where they pick up most of the cost also.