Airdrop is an opportunity to earn some cryptocurrencies without investing money. You have just to invest your time and do some work.
Some examples of tasks are:
1) Retweet post from ICO company
2) Share a post on your page on Fb
3) You can also promote their name on this forum etc.
You will be paid in their local ECR 20 cryptocurrency, then you can change it into real money. Nice opportunity to earn some money and learn from their projects.