A Crypto Airdrop is a free way to earn free coins from a blockchain's token or coins project. For example those blockchain project will send on your Ethereum address some free tokens that you will receive at the end of the airdrop event. Some other will ask you to own a specific token at a certain time on which they will create a snapshot and give you the same amount in their new token.
Crypto airdrops are essentially used as maketing campains, that will help for create awareness about the project. It is also a easy way to promote a project and build a crypto community. The crypto network effect is really strong as thousands of people can share the project's social medias.
You will have ways to get crypto airdrop. for example some will just ask to register on their site. On some other you will need to re-tweet one of their post on Twitter, register on their Telegram or post on your Facebook. Some will ask to post on forums such as Bitcointalk or even write a blog post.
Most of the time, Airdrops is safe. However there is also some fake crypto airdrops around. Please be aware and never give your private keys and never send your crypto to someone else in order to recieve your crypto airdrop.
In some rare cases, you will get your free crypto airdrops almost instantly. But for some airdrops you have to wait for some weeks, or months before receiving your cryptocurrency. This is because projects usually announce their airdrops before their ICO starts. Doing this give them the opportunity to grow their community and attract more customers. The airdrops are in most cases distributed after the ICO. You have to wait until all ICO stages end to receive your free crypto airdrops Don't be surprised if your wallet is still empty for one week after doing airdrops