A mystery shopper is normally used by companies to measure the quality of its service. Sometimes it is used to gather specific information about its products and services.
As a mystery shopper you would be asked to perform specific tasks such as buying a product, asking related or specific questions, and perhaps registering a complaint or being asked to behave in a certain way. You would then provide feedback about your experiences. More often than not experienced market research interviewers are asked to undertake mystery shopping but occasionally consumers participate as well.