What does open source mirroring mean? What's the use? Please be specific, thank you!

Open source mirroring refers to the mirroring server.

The service contents of the mirror server and the main server are the same, but they are placed in different places to share the load of the main server. It can be used, but it is not the original. Two or more servers on the Internet that have the same content and update synchronously are called mirror servers except the main server.