Nigel replied:

Repeaters extend the range of your radio transmission by receiving signals on one frequency and retransmitting them on another frequency. They are situated it higher locations so they get greater range. UHF repeaters transmit on channels 1-8 and receive on channels 31-38. ie a Channel 3 repeater receives on channel 33 and repeats the transmission on channel 3. To use a repeater you select the correct channel between 1-8 for a repeater that is in range, and make sure your set is in DUPLEX mode. Your set will then automatically transmit on the correct channel (31-38) but still receive on the set channel. The channel display will not change when in duplex mode. Keep in mind that if, for example, you are using channel 34 in simplex mode and you are in range of a channel 4 repeater then your conversation will be retransmitted by the repeater, much to the annoyance of anyone trying to use the repeater properly.