Visual line of sight (VLOS) means the remote pilot maintains continuous, unaided visual contact with the drone throughout the flight. Glasses or contact lenses are fine, but binoculars or a screen do not count — the view must be good enough to judge the drone's position and spot other air traffic in time.
VLOS is the default requirement of the whole open category: without it you may not fly in A1, A2 or A3. In practice it limits the flight distance — as soon as the drone becomes a dot whose orientation you can no longer tell, VLOS is lost.