Imagine three people spread out in open countyside. Assume person 1 and person 2 can hear and understand each other, and person 2 and person 3 can also hear and understand each other, but person 1 and person 3 are too far away from each other to hear or understand.
If person 2 simply repeats everything persons 1 and 2 say, then 1 and 3 can still communicate. This is not rocket science.
Now imagine the three communicators miles apart and using radios. 1 and 3 are still too far apart to talk directly, but 2 can hear both. Again, 2 can repeat what 1 and 3 say, allowing 1 and 3 to communicate.
Now, if 2 has two radios, he could hold the speaker of one radio to the microphone of the other. Then 1 could hear 3's voice directly, and vice versa.
A radio repeater does this automatically. It allows two or more people to communicate, even if they are out of range of each other. The repeater has a sensitive receiver and reasonably powerful transmitter, plus an antenna placed up high. This allows one or more of the users to use relatively small radios and antennas with low power. The repeater does all the "hard work".
In recent years it has become popular to link multiple repeaters together. If someone can reach one repeater, they can then reach them all.
There are several worldwide linked Ham Radio repeater systems. Some are linked by radio signals, some by telephone, and some by the Internet. Hams pioneered the idea of VOIP (voice over Internet Protocol) back in the 1980's over our own over-the-air "internet" long before the hard-wired internet and Worldwide Web became so popular.