Eye tracking allows testers to identify what participants look at during the course of a usability test. Eye tracking equipment uses several different technologies, including skin electrodes, marked contact lenses, image processing cameras, and reflector trackers.
Eye tracking involves sophisticated, usually expensive equipment.
In most cases, unless the particular product you're testing requires eye tracking testing, it's best to find a usability lab that has already purchased an eye tracker and rent time at that lab for your testing. Use this technique when you absolutely have to identify what a person looks at during a usability test. For most products, regular inspection or testing methods will identify enough usability problems to the point that eye tracking isn't necessary.