Polymorphism, a fundamental concept in object-oriented programming, means 'many forms'. In Java, it allows classes to perform the same action in different ways. This means that objects of different classes can be treated uniformly, responding to the same method call but with unique implementations based on their class. For example, an 'Animal' class might have an 'eat' method, but different animal subclasses (like 'Dog' or 'Cat') would implement the 'eat' method in their own specific ways, demonstrating polymorphism in action.