Maintenance fee is a more sophisticated name for domain renewal fee. In the past, .ro domains could be purchased by paying a single fee that offered the domain an indefinite period of operation. The new ROTLD regulations (entered into force on 01.03.2018) have changed this mode of operation, and the respective domains will have to be renewed or registered for a determined period, between 1 - 10 years. With the completion of this registration period, the domain will have to be paid again to ensure its operation. If you receive such a message, you will have to check where the domain for which you receive this message is registered and make sure in advance that you have the possibility of renewing or transferring it to another registrar. In the case of .ro domains registered or transferred to Hostico, as for the other services, the renewal invoice will be issued two weeks before the due date. If you want to transfer the .ro domain to Hostico - the steps to follow are detailed in the tutorial: .ro domain transfer
You can read more information about the new ROTLD regulations at the link: ROTLD annual maintenance.