What does "communicating" mean in safe driving?

Other drivers may not notice your vehicle, even when in plain sight. To help prevent accidents, let them know you are there. When Passing. Whenever you are about to pass a vehicle, pedestrian, or bicyclist, assume they do not see you. They could suddenly move in front of you. When it is legal, tap the horn lightly or, at night, flash your lights from low to high beam and back. Drive carefully enough to avoid an accident, even if they do not see or hear you. [Commercial Driver Handbook]