I don't get the safety slogan. If I see it, flee; but if I hear it, clear it? Clear what?

"If you can see it, flee it; if you can hear it, clear it." Could you explain the saying? I am confused.

I was at the race. The PA system cannot be heard away from the stands, in the food areas and bathrooms. In any event, announcements were made for fans to return to their cars however getting 85,000 out of the track and to their cars is not an easy feat, nor safe in a thunderstorm. People just have to use common sense. We saw the approaching clouds and storm (the second storm that hit around 5PM) and heading for our car before the storm hit.

Lighting did not kill anyone, lighning did