When a sub class inherits the properties of super class , then we can override the method of super class.
This will acquire the properties of super class but can define its own behaviour.

In general way, we can say the overriding means to override the functionality of existing method.

Example:

class A{
    public void test()
    {
        System.out.println("This is my Super Class");
    }
}
class B extends A{
    public void test()
    {
        System.out.println("This is my Sub Class");
    }
}
public class testOverride
{
    public static void main(String a[])
    {
        A a1 = new A(); // class A reference and object 
        A a2 = new B(); // class A reference but class B object
        a1.test(); // this will execute the method in super class
        a2.test();// this will execute the method in sub class

    }
}

In the above program, method test() in class B override the method test() in class A, therefore when declare an reference a2 of class A, it will execute the method in class B.

When we compile the above code, there will be not error as both the class have method as test(), but in runtime JVM will find the object a2 is of class B and will execute its own method.