airdrops is a free token/ coins given by an ICO to help their project to be recognize. It helps a newly launch ICO promote their project and gain more investors. There's not much requirements on participating in an airdrops, it's just simply filling up a form with you wallet details etc..but some airdrops requires joining their twitter account and retweet to help promote the project. But I guess, these days some airdrops is not that profitable and collecting them is just a waste of time. The sad part is there are airdrops that also scam people.