Let say you and a friend are out in the woods. You can reliably communicate using your own voices and ears up to 200 feet apart. That would be similar to how simplex radio works. Your radio can only transmit so far to the other radio.
Now, bring in a third friend. Have him stand where friend 2 was, and now have friend two walk another 200 feet away. You can now talk to friend 3, 200 feet away from you, and he, in turn, can talk to the other friend who is now a total of 400 feet from you.
Now put said friend on a mountain top with a hearing aid and a mega phone. Sorta a simple explanation about how a repeater works.
The limitations of your mobile or handheld radio are:
Limited transit power.
Limited gan antenna.
Limited range due to topographical shielding (fancy way of saying "there is a dang mountain in my way)
While it may be difficult to talk to a radio 5 miles away due to the above limitations, it is a lot easier for your radio to be heard from a mountain top, from the top of a tall building, or the top of a tall tower.
The repeater can run a larger antenna with more gain (able to hear quieter signals)
The repeater can run more power (Can yell louder)
The repeater is on top of a hill, so it can "see" over the mountain that is in your way. (can yell around corners)
The repeater also breaks down at the least opportune time, usually in the rain or snow, and almost always in the middle of the night. You will rarely have the correct part in your truck, but that's a discussion for another time...