The airdrops are free tokens which some of the companies give away during their ICO's. Usually in order to receive those free tokens you need to register at their website. After you will be offered to perform some simple actions such as subscribing to social media or filling out a form with some of your data. After you complete with these steps, the system will issue free tokens for you.

If you participate in many programs of that kind, you may be able to get some real money out of it.