Interface fees refer to the fee charged for using a particular interface to access and interact with a service or product. In this case, the interface fee is a fee charged for providing a farming interface, which allows users to access and manage their farming rewards. This fee is typically used to cover the cost of maintaining the interface, as well as any additional costs associated with providing the service. By charging an interface fee, users are able to access and take advantage of the benefits of the farming interface, while the provider of the interface is able to cover their costs.