What does a \"Wig Wag\" means?

A A mechanical arm that signals low air pressure.

explanation

Another type of warning is the “wig wag.” This device drops a mechanical arm into your view when the pressure in the system drops between 55 and 75 psi. An automatic wig wag will rise out of your view when the pressure in the system goes above 55 and 75 psi. [Commercial Driver Handbook]