They give you a discount per month. So for example, a $1000 phone is on sale for $600, but they don’t give you $400 off up front. Its a $11.11 per month discount for the next 36 months. You pay $16.67 per month until its paid off. If you pay it up front you pay $1000. If you leave early or pay it off early you lose the $11/month discount on the payment for the phone

Its intended to lure people in for an advertised cheap or free phone, but then you’re stuck with them paying for the device.