Interfaces allow multiple classes to implement the same method signature, enabling polymorphism.
Interfaces define methods that multiple classes can implement, allowing them to be used interchangeably. This is a foundation of polymorphism, enabling a single interface reference to point to objects of different classes. For example, both 'Order' and 'ServiceOrder' could implement an 'Orderable' interface, allowing functions to handle both types through the 'Orderable' interface.