Solo - PFL

Mixing consoles always have the ability to 'solo' any channel so that you hear
than channel by itself through the monitors, generally without affecting the
signal being recorded (or amplified or broadcast).

This can operate in a variety of ways. A large-scale console will give you
options, on a smaller console it will probably be fixed.

• PFL: Pre Fade Listen means that you hear the signal of the selected channel
  alone, picked off from a point before the fader, so that the fader level has
  no influence on the PFL level. On some consoles, the PFL signal is routed
  equally to left and right speakers, on others the position of the pan control
  is retained. PFL is often used for setting the gain control: on each channel,
  one at a time, press the PFL button and set the gain so that the main meter
  reads a good strong level without going into the red.

• Solo or AFL: Solo, or After Fade Listen, is as above, but the position of
  the fader is taken into account.

• Solo-In-Place ("SIP", "Cut Solo" or "Kill Solo")
  mutes all other channels. Exceptionally, this does affect the other outputs
  of the console, which is why you will never find it on a live broadcast console.

• Solo Safe: Often it is useful to protect some channels so that even when
  other channels are soloed, they still operate. This is of greatest benefit
  with Solo-In-Place, where the channels used as effects returns can be placed
  in Solo Safe mode. Now when you solo a channel, you will hear it in isolation,
  at its correct level, correct pan position, and with its reverb intact.