When you run a host, renters form contracts with you. Each time that you make a new contract, you have to put coins into the contract that can be taken away from you if you are a bad host. Depending on how much collateral you are offering, you will have to put up between 5 and 500 coins per contract that you form. You will get all these coins back (and more) if you keep the data that the renter gives to you. And if you lose data, you could end up losing these coins.