This phrase is often seen on the back of semi-trucks and means that if you can see the truck's mirrors, you are too close and should slow down. This is because semi-trucks have a large blind spot directly behind them, and if you are too close, the truck driver may not be able to see you and could accidentally run into you.
The phrase is meant to remind drivers to stay a safe distance behind semi-trucks and to be aware of the truck's blind spot. By doing this, drivers can help to prevent accidents and keep themselves and others safe on the road.
