A Facebook pixel is a hidden piece of code that sits in the back end of your website. The pixel tracks who visits your website, which pages and products they view and what actions they do or don’t take. This allows you to effectively run Facebook ads to people based on what they have and have not done such as added to their cart but not purchased or registered their email but not booked for an event. You can target a highly warm audience and save money by excluding people who have already taken a desired action.