Polymorphism is consitituted with two words, Poly and Morph. Poly means multiple and Morph means Shape. So if same method of an object can be used at multiple level depending upon the situation faced(parameter passed) at run time. Let me explain you with a simple example, Suppose a method named as move(). This method will react differently on different object. For example, for Fish its ation will be swim, for Bird its action will be fly, for Dog its action will be run.... now for a particular object depending on parameter passed the further reaction will take place... Suppose if you pass "Bread" parameter to move() method of Dog, obviously it will move toward Bread... and if you pass "Stick" parameter to move() method of Dog, it will run away... Similary way in
Java, same method will be used at different situation depending on parameter passed and over object called.
I hope this will further clarify the doubts.