what is the functionality of use of "Airdrop" in smart contract, have seen many where result pop-up "adding airdrop in already deployed smart contract" as far as my knowledge once smart contract is deployed can't make any change so how "Air Drop" can be added as a functionality ?
Airdrop is the process to send tokens in an automated manner.
For example you have a pre ICO, and people contribute to a normal address. When you finish your ICO contracts you want to allocate tokens to the early contributors.
Another purpose is as marketing tool, you randomly select some users and send them some tokens.
An airdrop doesn't require an special contract and can be done through a standard ERC20 contract. But there are different versions that try to minimize costs.