Projects use airdrops to introduce their tokens to the crypto community. They give away a certain amount of their tokens for free. This means they can gather a big community in a much easier way than if they had to find buyers for their tokens. The airdrop thus benefits their ICO.
Usually it is enough to just register on their page or joining their telegramm group (instructions can normally be found on the website of the airdrop). Sometimes there are other tasks involved like retweeting a message. Airdrops are always free to participate in.
To round it up, you basically just have to sign up for them.
For a detailed instruction please visit our F.A.Q. Section