Final answer:
A large letter "X" signifies that the runway is closed, marking it unavailable for landings or takeoffs due to reasons such as construction or maintenance.
Step-by-step explanation:
A large letter "X" placed near the runway threshold indicates that the runway is closed. This is an important safety feature in aviation as it communicates to pilots that they cannot land or takeoff from that particular runway.
Runways can be closed for various reasons, including maintenance, construction, or emergency situations. When you see such a marking, it unequivocally means that the runway is not available for use and attempting to use it could be dangerous.