is-a keyword

sir , what is the use of is-a keyword.

is-a is NOT a keyword! It's a basic English term used to describe whether a given class is a sub-class of another class.

For example, consider this:

class Coffee{ }

class Cappuccino extends Coffee { }

Now, in English, we can simply say that "Cappuccino is a Coffee". In other words, we can describe this relationship as that "Cappuccino shares an is-a relationship with Coffee".