Google uses the app (available on the Google Play store) to send you short surveys occasionally. You will not get a lot of Play credit for participating, and the amount of of questions you are sent seems to depend on your demographics. Anecdotally, it seems that affluent females receive a larger number of questions. Also, if you have locations services enabled on your mobile device, it seems that you are more likely to get questions based on your actual shopping history.
There was a discussion on reddit recently about this. From reading through the comments, it looks like the average male user was seeing about $10-15 in credits after a year of participating. It was higher than that for females, with one reddit reporting his 18-24 female girlfriend has made $53 in the past year. Obviously this is also anecdotal, but it gives you an idea of what to expect.
If you want to try it, the app is here: https://play.google.com/store/apps/details?id=com.google.android.apps.paidtasks&hl=en