For a repeater to be able to "repeat" your transmitted signal, it has to do so on two different frequencies. The frequency listed for that repeater will always be the frequency you receive from that repeater. Most modern radios will have ARS (automatic repeater shift) programed into the operating menu, so if you program in the repeater's transmit frequency, it'll automatically add in the 600 Khz shift for you.
When you press the PTT on your radio's mic, the radio will automatically shift 600 either + or - and transmit your voice message into the repeater's receiver, and at the same time the repeater will re-transmit that voice message back out on a 600 Khz shift to all other radios programed to that repeater's receive frequency.
You don't hear your own transmission because during the time of your transmission, your radio is either + or - 600 Khz away, once you release the PTT, your radio returns to the repeater's receive frequency so you can instantly monitor a response to your voice message.
I sure hope I didn't confuse you more on this......:redface: