Pixel trackers are invisible images which are the size of a single pixel hidden within a given email (Not visible in most mail clients). These images are sourced from a unique url (unique per email address) which when accessed are tracked by the web service as a view of a given email from a given individual. These are usually used in marketing emails to see how effective their campaigns are.

more info @ https://en.ryte.com/wiki/Tracking_Pixel