Please clearly tell me what is overriding ?

Answer 1

A method of a superclass is overriden in the subclass when the subclass creates its own implementation of the method.
Example (Java):
public class SuperClass {
public void methodToOverride(){
System.out.println("Original implementation");
}
}

public class SubClass extends SuperClass {
@Override
public void methodToOverride() {
System.out.println("New Implementation for SubClass");
}
}

public class Main
{
public static void main(String[] args){
SubClass a = new SubClass();
a.methodToOverride();
//output is New Implementation for SubClass
}
}

Answer 2

Overriding in general terms means changing the behaviour of a predefined function and changing it for a certain context. This is useful in a number of ways, such as defining different behaviour for subclasses or allowing custom classes to use the +, -, *, / and other operators.
Not to be confused with overloading, which is making two functions with the same name but different parameters.