A transceiver's tone squelch circuit mutes or silences the audio output of the receiver's speaker in the absence of the desired radio signal, or tone.

Some repeaters transmit a CTCSS tone on the output (downlink) signal. Your radio can be programmed to ignore all other incoming signals that are absent this tone. This helps to keep the receiver audio quiet, especially when in a high-noise environment where other unwanted signals and transmissions may be received. Oftentimes, these unwanted signals are heard as white noise, or static.

In amateur radio parlance, this type of circuit can be referred to as PL decode, PL squelch, tone decode, tone squelch, CTCSS decode, or CTCSS squelch. In RepeaterBook entries, the tone transmitted by the repeater is called the downlink tone.