Plain English Meaning
LF/MF is a low end of the radio spectrum where some older navigation beacons transmit. When a chart or handbook says a navigation aid is 'LF/MF,' it means the signal is on a long-wavelength frequency that an ADF receiver in the cockpit can pick up.
Full Definition
A grouping of two adjacent radio frequency bands used by certain ground-based aviation navigation aids. Low Frequency (LF) covers 30 to 300 kHz, and Medium Frequency (MF) covers 300 to 3,000 kHz. In aviation, the LF/MF is most commonly with Non-Directional Beacons (NDBs) and the receivers ( Direction Finders, or ADFs) that with them.