You download the app and sends you surveys. They pay you for the surveys in Google Play Store credit. You can spend that on apps, in-app purchases, etc.
Qmee, Attapoll, Survey Spin and Leger opinion. It's a tedious way to make money and you get disqualified regularly but I really have earned a lot just pegging away. .
It's not much, but it's something. Of course the more questions you answer within a single survey the more credit you get. It'll ask "have you visited -list-recently?" and it will ask about your experience at that location. Or if you googled something it'll ask if you found the right results. But be careful, it does throw in dummy questions to see if you're lying, which is the $0.10 surveys.