Final answer:
A flashing red light signal from the control tower means the airport is unsafe for landing, and the pilot should abort the landing.
Step-by-step explanation:
When an aircraft is on final approach, a flashing red light signal from the control tower indicates to the pilot that the airport is not safe for landing. It is a warning for the pilot to abort the landing attempt and go around. This is a part of the light signals used for ground-to-air communication, especially in situations where radio communication is not available or when the aircraft is not equipped with a radio.