From Kg4ivd's wiki

Also known as PL or CTCSS, is a method to descriminate against unwanted signals.

CTCSS works by looking for a sub-audible tone to the signal. If signal does not have the tone, it will be ignored; all signals for that reciever must have that tone injected into it.

An example of a place where this is very usefull is when two repeaters are close togeather, for example one in Raleigh and one 100 miles away in Greensboro, and both have the same input frequency. Normally, the two are far enough away that there is no problem, but if a perticularly strong station is in the middle or a band opening the signal might make it to both repeaters. If the Raleigh repeater required a 110.9 Hz tone and the Greensboro repeater required 82.5 Hz tone, then only the repeater that the transmitting stations wanted to contact would repeat the signal!

CTCSS Frequencies

This is a list of frequencies commonly used as CTCSS tones. All tones are in Hz.

67	71.9	74.4	77	79.7	82.5	85.4	88.5
91.5	94.8	97.4	100	103.5	107.2	110.9	114.8
118.8	123	127.3	131.8	136.5	141.3	146.2	151.4
156.7	162.2	167.9	173.8	179.9	186.2	192.8	203.5
210.7	218.1	225.7	233.6	241.8	250.3