Polymorphism means "many forms." In Java, it means a single variable can refer to objects of different classes, and method calls on that variable behave differently depending on the actual object.
Imagine you have an Animal variable. At different points in your program, it might hold a Dog, a Cat, or a Bird. When you call speak(), the correct version runs based on what object is there, not what the variable type says.
Without polymorphism, you'd write separate code paths for every animal type. Your methods would need if checks: is it a Dog? A Cat? Every new type forces you to update every method. Polymorphism removes that. You write one code path that works with Animal, and Java figures out the rest.