What does it mean when a bus drivers flashes his headlights at you?

It's not just bus drivers, it's any driver. It can mean several things. If you are driving in hard to see conditions (dusk or dawn, rainy. hazy, night time) and you are driving without your driving lights on- it means turn your driving lights on. But, most often it means there is some kind of patrol officer ahead, so if you are speeding you might want to slow down to the posted speed limit.