The repeater is designed to receive signals on one designated channel and re-transmit them on another channel.

Repeaters are usually located in prominent locations (on hill mountains or tall buildings), so that their receive and transmit ranges are greatly increased  over normal base or mobile installations. This allows the repeaters to receive and transmit signals which would  normally be out of range of many base or mobile transceivers in the area. 

UHF CB repeaters in Australia are designed to receive signals on channels 31-38 and retransmit them on channels 1-8 (i.e. 30 channels lower) .

Normally, uhf transceivers transmit and receive on the same channel. This is known as SIMPLEX. However, to communicate through repeaters, you transceiver must  be able to transmit on a different channel to that which it receives (known as SEMI-DUPLEX, or DUPLEX for short).