Talkback lets you hear your voice when you transmit. Your voice will come through the speaker in the CB, or through an external speaker if you have one plugged into the radio.
Why would you want to hear your voice? Talkback lets you monitor exactly what other people are hearing when you transmit. If you have a special accessory like an echo microphone, for example, Talkback lets you hear the amount of echo effect so that you can adjust the echo accordingly.