Simply put a repeater is an automatic receiver and transmitter located at a higher elevation than usual.

Radio waves travel in straight lines and the paths is attenuated by obstacles such as buildings, people and trees. Locating the repeater at a higher altitude provides a clear path avoiding these obstacles and improving both the receive and transmit range.
The higher the repeater the greater the coverage area.

Since it’s not possible for the repeater to receive and transmit on the same frequency simultaneously, two frequencies are used.
As an example the 2 Meter K6EAG repeater transmits on 145.130 MHz and receives on 144.530 MHz. This means that your transceiver needs to be the reverse, receiving on 145.130 and transmitting on 144.530 MHz.

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