What does it mean when the phone has clean esn?

The ESN is the phone id that tells the cell towers where to send the calls. If someone decides not to make payments on their phone bill, the look at that esn and cancel it until they pay up. If they dont, they might want to sell that phone as is. So when you buy that same phone, the company will look at that esn and see money is owed and expect you to pay for it. So a clean ESN would be someone that doesn't own any money on that phone service and you dont have to worry about paying other people's bill.