What: It's a two-way radio system that receives on one frequency, then re-transmits what it receives on another frequency; at exactly the same time. It's nothing more than a "dumb electronic machine" with some smart people behind it.

Why it's needed: Your mobile or handleld transceiver, has a limited range due to it's antenna height with respect to the radio horizon and rf attenuating surroundings. Repeater systems are used to "transfer" your transmitted and received signals to much higher levels electronically using large, very efficient high gain antennas, low loss feedlines and a transmitter and receiver that is rated for heavy or continuous duty. A repeater "gets out" your signal and receives the station you are talking to with a far greater range and coverage area! You take advantage of the repeater's higher elevation to increase your effective transmitting and receiving coverage versus your mobile or hand held transceiver!
Simple diagram below: