VLF stands for Very Low Frequency, a radio band used for long-range transmission. Because these waves travel well over large distances and can be received by simple antennas, they are often used for navigation, submarine communication, and time-signal broadcasts such as DCF77.
In cyber security, VLF matters because any system that trusts an external radio reference is depending on an outside source for correctness. If that signal is weak, noisy, jammed, or spoofed, a device may drift, misreport time, or make unreliable decisions based on bad synchronization. Defenders reduce this risk by validating the received signal, comparing it with other time sources, and keeping a local holdover clock so the system can stay stable when the broadcast is unavailable.