1) The Midland radios have the ability to enable the 8 extra channels for repeater use. These will transmit on 467.xxx MHz and receive on 462.xxx MHz, whereas the simplex (direct) channels will transmit and receive on the same 462.xxx MHz frequency. In addition to the frequency a repeater transmits, there is also the CTCSS tone (also known as PL tone) which allows a repeater to selectively repeat transmissions from one group of users when others may be on the same shared frequency.
On more advanced radios, you would set up a channel for each repeater in the radio's memory, where they would each be one of the 8 repeater channels (i.e. 462.550 MHz), but the CTCSS tone would vary for each repeater. On the Midlands, you need to manually change the tone to use a different repeater.
So in the example above, you'd tune to channel 15R (repeater channel) and set the tone in the menu to whatever the local repeater on Channel 15 (462.550) requires. If you go to a new area where there is another repeater also on 462.550, you would stay on Channel 15 but change the tone to whatever that new repeater uses.
2) A repeater will extend the range a single radios has by many miles, depending on the height of the repeater in elevation. If two radios are nearby but suddenly cannot communicate due to the terrain or distance, a nearby repeater located in a tall spot (a tower, a tall building, or a mountain top) will allow those radios to communicate by retransmitting the signal at higher power and at a higher location. UHF frequencies are almost line-of-sight, where two stations can communicate if there is an unobstructed view between antennas.
By putting the repeater antenna high up, it will be able to "see" a much larger area and thus extend the mobile units' coverage over the greater area it can "see" from its height. So a radio with a 1 mile range can suddenly get 15-30 mile range if there is a repeater in a good spot high above the average terrain height by having the repeater retransmit its signal at a higher power and elevation. Repeaters have a limit to their range based on this elevation, as the visual horizon distance changes with regard to height above the ground.
Since the Earth is curved (flat-earthers will be disappointed), the further away you go from a station, you begin to curve below the horizon and eventually the Earth itself will block the signal. The only way around that is more height, and that's how satellites can have such wide coverage. They are essentially a repeater at an extreme altitude and thus they have visual line-of-sight to a much, much larger area than a tower on the ground could ever have.