Air Traffic Control. The system of people and equipment designed to help keep you safe in the air.

Per FAR 91.125, light gun signals that ATC can send you in the event of lost comms are:

Color/Type On Ground In Flight
Green, Steady Cleared for takeoff Cleared to land
Green, Flashing Cleared to taxi Return for landing
Red, Steady Stop Give way to other aircraft and continue circling
Red, Flashing Taxi clear of runway in use Airport unsafe - do not land
White, Flashing Return to starting point on airport N/A
Red & Green, alternating Exercise Extreme Caution Exercise Extreme Caution