Airdrop is the devs pretty much giving you a free amount of coins, sometimes depending on how much BTC you have in your wallet, just so they're sure you're not just some guy who made multiple bitcointalk accounts to claim airdrops multiple times.
If you see the words "airdrop", run. It's always a scam. And when it's not, it's still a scam.
Definitely not. How is airdrop a scam? Whether or not the coins are worth anything or do anything at all, they don't ask you to pay for anything, they just give you coins for free; hence the name "airdrop". Airdrops are only a scam if they ask you to hand over your wallet's private keys or something similar.