What is Airdrop?

Updated 

An airdrop is an unsolicited distribution of a cryptocurrency token or coin, usually for free, to numerous wallet addresses. This is distributed in a specific ratio, i.e. 1 free tokens for every 100 held at 23 Feb 2023 16:00 hours. 
This is usually done by taking a snapshot of the tokens balances which you hold at the snapshot specific time.