From The Guardian
Metadata is information generated as you use technology, and its use has been the subject of controversy since NSA’s secret surveillance program was revealed. Examples include the date and time you called somebody or the location from which you last accessed your email. The data collected generally does not contain personal or content-specific details, but rather transactional information about the user, the device and activities taking place. In some cases you can limit the information that is collected – by turning off location services on your cell phone for instance – but many times you cannot. [E]xplore some of the data collected through activities you do every day.