In Java, an interface defines a set of methods that a class must implement if it wishes to adhere to that interface. An interface specifies the method signatures, but not their implementation. Essentially, an interface acts as a contract between the class that implements it and any other classes that will interact with it.

Interfaces allow for classes to have common behavior without needing to be related by inheritance. This is useful because Java does not allow for multiple inheritance with classes, but allows for multiple inheritance with interfaces.

By implementing an interface, a class can indicate that it supports a particular set of behaviors, regardless of its specific implementation. For example, the Comparable interface in Java allows objects of a class to be compared to each other. A class that implements this interface is indicating that its instances should be able to be sorted in some way.

In summary, interfaces in Java allow for an abstract definition of behavior that can be implemented by multiple classes, promoting modularity and flexibility in the code.