It literally means what it is. They "airdrop" their coins/tokens to people who sort of sign up. Most of the time you just need to prove ownership of a bitcoin wallet and they give you a percentage depending on how much
BTC you hold on that specific wallet. To prove ownership, you send them a specific amount of
BTC or you can sign a message using your wallet[1] instead if you don't want to spend your
BTC.
[1]
https://medium.com/@TenX/mycelium-how-to-sign-a-message-with-a-bitcoin-address-87afdd0a5a0e If one is required to make a given payment for it, do you still qualify it as airdrop?