Why Use A Repeater?
Repeaters are used where direct line of sight communication is not possible between all the stations needing to communicate with each other. A repeater contains a receiver and a transmitter, these are set to two different frequencies. The receiver receives a signal on the repeater's input frequency and passes it to the transmitter which transmits it on the repeater's output frequency.
Below is a simple diagram showing that direct line of sight communication is not possible between the two stations but they can both communicate with the repeater.