As of a few months ago the Stillwater Amateur Radio Club VHF repeater on 145.35 requires a 107.2Hz PL tone. People from Missouri kept getting into it, I'd guess someone up there has a beam and a lot of power. Or maybe it's someone close by trying to get into the Missouri repeater and junking up Stillwater off a secondary lobe. For the 99.9% that aren't radio operators the PL tone is an audible sine wave at a frequency lower than normal speech. On a piano 107Hz is close to an A below low C - an octave below middle C. It's low but not as low as a 60Hz electrical hum. It's woofer but not subwoofer territory. Most human speech is between 300Hz and 3,000Hz so ham radios tend to filter out anything less with a 300Hz high-pass filter. But this filter is in the audio section - in the audio amp. The radio still transmits lower frequencies, the radios just make them effectively subaudible. So you can add these tones and program radios to make them do things. A repeater is a pair of radios, a receiver and transmitter. The receiver listens to a certain frequency and the transmitter retransmits whatever the receiver hears (but on a different frequency. It can be set up to retransmit everything, or to only transmit when it hears a specific frequency - the PL tone. If it's doing this it will ignore any signal that does not include the tone. So someone from Missouri can't accidentally have their signal retransmitted across Oklahoma.