Although you might not know it, your eyes are constantly moving, making quick adjustments to take in details. Only the very small, central portion of the eye, called the fovea, is capable of really seeing details. As you look at something complicated, like a dating profile, with different text elements, photos and advertisements, your eyes make rapid movements so the fovea can briefly focus on each of the different elements that catch your eye's attention. Each tiny focusing event is called a fixation.
Because the eyes make this tiny movement when we study objects or read, the eye-tracking systems can figure out what we looked at, precisely, and for how long. The image above shows the gaze fixations of a man in the first 10 seconds that he looked at the profile, which totalled more than 200 fixations.