As a mystery shopper, it's your job to visit a shop or restaurant, make some calls to helpdesks, use services, ... and fill out a survey about it. You'll always get a sum of money on completing the survey. This fee is usually somewhere around € 15. Sometimes the fee is a lot lower, but then you'll get some other benefit such as a free meal in a fastfood restaurant.
You may think it's easy money, but don't underestimate the length of the survey. You'll have to pay attention to lots of things. Since you're not allowed to take the survey with you on your mission, you'll have to memorize the most of it.
So even though you'll have to put a little effort in it, it's a relative easy way to earn a little extra money. Especially when pick a store you're about to visit anyway.