It's usually something done by new coins, where you can "apply" for the airdrop, and if you meet their certain list of requirements, then you are eligible to receive free coins or tokens by them

Typically, these requirements involve you having an active profile on the native coin's forum, or on a separate forum. The reason for this is that shows that you are sharing this new coin with a wider community, and possibly on a more popular forum.

For example, I have seen coin airdrops that require for you to have an active profile on this forum, as it is so popular so that you can promote their coin to other users who share interests in crypto-currencies.

In shorthand, it is the makers of the coin "giving away" coins to certain user's for free in order to spread awareness.