While running, it constantly monitors modem channel 65533 for incoming rednet messages, and re-sends anything it detects (as of CC 1.6, all messages sent via rednet.send() are automatically distributed both on the channel the coder specifies and on channel 65533, making it possible for repeater systems to pick them up without monitoring the entire channel range).
Bearing in mind that the wireless range of a given system will be higher the closer it is to world height, and that two-way communications are possible between systems so long as at least one of them has sufficient range, one repeater system placed at max altitude can provide coverage between quite a large number of systems down below that otherwise might not be able to reach each other. If an Ender Modem is used, then a single repeater system will typically cover the needs of an entire server.
The system running the repeat script will maintain a visible running tally of how many messages it has repeated.
If you wish to have a system automatically boot the repeat script every time it becomes chunk-loaded, you might create a startup script with the following content:
If you wish to continue using a system while it's acting as a repeater, consider firing it up in a new multishell tab: