Eye-tracking systems use an infrared camera, often mounted below or built into a monitor, to follow where a user is looking and translate that gaze into cursor movement, selection, and typing. They are widely used by people with severe motor disabilities, including conditions like ALS, who cannot reliably operate a mouse, keyboard, or touchscreen.
For web content, this means interactive elements need to be large enough and spaced widely enough to select reliably by gaze, and pages should not depend on hover-triggered content that eye-tracking users would activate simply by looking, since a gaze cannot easily hover without accidentally selecting.