Once upon a time, you may have had a close friend recommending a book, song, or movie to you – since the friend knew your “tastes,” you would typically check out their recommendations.
In the online world today, websites and mobile apps (and the companies that build them) have collected data about all their visitors and customers at a granular level of possibly each click that has happened on the website/app. This data includes every book/song/movie/product they have purchased/rejected ...