It is a marketing campaign that refers to the accelerated distribution of a cryptocurrency across a population of people. It usually occurs when the creator of a cryptocurrency provides their coin to lower tier merchants or existing community members to increase their usage and popularity.
They are usually offered for free or in exchange for simple tasks such as sharing news on social networks, or you have to join a community on Telegram.
The website airdrops.io aggregates the lastest available and potential aidrop.