I’m cube-bound - what did you mean 5s free?
It means when you initiate a new two year contract on any of the cellular phone carriers, you can select the iPhone 5s for free. . . but you will pay for it with your contract over the two years of the use of the cellular service. There is no initial fee except perhaps a connection fee, usually about $35 which you can often get them to waive. The others are last year's iPhone 6 16GB for $99, 64GB for $199, and 128GB for 299, then last years iPhone 6 Plus 16GB for $199, 64GB for $299, and 128 GB for $399, all with two year contracts. The newer models soon to be released are, with a two year contract, are iPhone 6s 16GB for $199, 64GB for $299, and 128GB for $399, 6S Plus 16GB for $299, 64GB for $399, and 128GB for $499. Unlocked with out contract are a lot more.
Apple will allow you to buy an iPhone 6s with AppleCare + and exchange it every year, starting at $32 per month for the lowest 16GB model. . . just choose your carrier. You then pay for service over that amount. Every year, you come in and turn in your last year's model and pick up a new one. Cost is $384 per year minimum, paid monthly. . . but you get Apple Care+, a $99 for two year value.
Or, you can buy your iPhone using one of the carriers' similar monthly payment programs. AT&T calls theirs the NEXT program. They run between $27 and $37 per month, IIRC.