It’s a contract (mutual agreement) between the client code and the implementation of the interface. The implementation fulfills certain obligations provided the client code fulfills certain other obligations. The interface defines what those obligations are. If the client code fails to meet its obligations, then the implementation of the interface isn’t bound to fulfill its obligations anymore either. The point is to think in terms of two parties, where the developer who is either using or implementing an interface takes on the role of one of the two contractual parties.