Interfaces are a way to provide a basic blueprint for a class -- they specify the fields and methods a class implementing a particular interface is expected to have.
It's a bit confusing because interfaces can't be instantiated into an object like a normal class... instead, when you're making a new class, you can say that your class 'implements' one or more interface. This means that the class you're creating will be responsible for having all of the fields and methods specified by the interface, but this also means that your class can be used interchangeably by other code that might be expecting objects with particular fields or methods.
Since we don't have multiple inheritance in Java, there are cases where implementing several interfaces in one class makes sense.
