Eye Tracker is a technology or device that monitors eye position and eye movements as a means “of detecting a things” or “of studying how people interact computer, equipment, mobile phone, etc.” (while reading text, using apps, working on something).
Eye tracking is the process of measuring either the point of gaze (where one is looking) or the motion of an eye relative to the head. (cr: wikipedia).
The Eye Tracking Process is useful for research that it helps inform what/when users are doing in particular situations or designed features (of the text position, icons, directions, and much more). There are many applications for, to name a few, cognitive science, psychology, human-computer interaction(HCI), visual spatial cognition, architecture, virtual reality, and/or marketing research.
Link here shows a brief information from Tonii Technology, one of the technology companies providing the tracking equipment.
image credit: ergoneers.com & Tobii.com