The purpose of a Repeater is to listen on one frequency, and transmit what it heard onto another frequency.  We will talk about it from the perspective of the radio you are using (and not the repeater itself)  The main frequncy that to program is the one your radio is listening to.  The offset determines the frequency your radio will switch to to transmit.  Why bother having two different frequencies, you may wonder?  Well, think of each frequency as a car.  On one side to hit your Push-To-Talk button to transmit.  One the other side the repeater hears you, turns on, and is repeating everything you say (hence the name repeater).  So you have two (2) transmissions going on at the same time.  If they where on the same frequency, or road-lane, you would have a collision.  This would cancel out each transmission.  Well, there is also that "Tone" or "PL" that one has to program into there radio.  This acts like a key, and turns it on.  These tones are too low to be heard on most communication speakers.  One does need it if the repeater requires it.  Not all use a tone.  The reason for it, is to limit the amount of intereferrance it receives.  In areas like cities, there are all kinds of souces for RF. Any number of them can turn the repeater on, even though nobody is using it.  This can become VERY annoying.  To minimize how much this happens, they use a tone.  This acts like a key, to turn it on, and off.