The squelch is the function on the radio that keeps you from hearing static between transmissions. If you turn the squelch level down to 0 on your radio, you will hear static and that gets old rather quickly. Turning the squelch level up, eliminates that static and you only hear something when someone transmits - UNLESS - you happen to be in an area with alot of interference. Sometimes electronics transmit RF (radio frequency or radio signals) that they arent intending to, and sometimes you can hear that on your radio. Adding PL to the RX side of your channel helps to eliminate that - it will only open up the squelch if a legit signal transmitting with that PL is heard. This can be used on simplex frequencies as long as your buddy is setting the same PL tone on his TX/RX (think "Privacy Channels" by Midland), but it's usually used in conjunction with repeaters. I hope my explanation makes sense!