RingPlus is one of the few carriers which offer a truly free cell phone plan.
Most RingPlus plans are either free or very low cost and sometimes require you to pay a "Top-Up" charge. The charge is to cover your expenses in the event you use up your monthly allowance - rather than blocking service they will start to use this to allow you to keep using your phone.
If you go over your monthly included allowance of dat, voice or text, the $15 would start to be used. At this point, you'll be responsible to add another $15 to your account in the event it occurs again. The "Top-Up" basically acts as a security for the service provider to make sure they are not stuck with the bill.
To be eligible to use RingPlus, you'll need to have an unlocked Sprint device.