What is airdrop?
Airdrop is a free distribution of tokens or coins to everyone (of course in a limited volume). Usually, airdrops are held to popularize some new coin that has recently entered the exchange or is being sold in ico, or an old coin that is losing popularity. The creators also can conduct the airdrop this way: you keep a certain amount of coins in your wallet, and you are given a portion (10% or 20% for example) of the amount of coins you have in your wallet. Airdrop allows creators to make the coin very popular in a very short time.
How to receive the airdrop?
In order to receive free coins, you need to register in such social networks as Twitter, Facebook, Reddit, Telegram, Medium, Bitcointalk. On Twitter, you usually need to subscribe to the channel, like the post and retweet it. In Facebook you usually need to subscribe, like (page and post) and share the post. In general, you are required to simple actions in social networks. When you fill out an AIRDROP form, you also need to enter your ETH(rarely NEO, WAVES, XLM) address and your email (you will receive a letter in the mail that you need to confirm).
Estimated value of airdrops
Estimated value of airdrops can be different: some give only 0.1$, while others can even give 100$. I recommend you to hold airdrop coins because, for example, coins worth 0.1$ can increase many times, for example, to 5$.