Maintenance Mode is a small server side utility that allows you to place your server in Maintenance mode. During this time, only players you choose will be able to access your server, and everyone else will be disconnected.
This utility is aimed at server owners to allow them to perform config changes, test new mods or configure new server features, without people randomly joining, or having to de-whitelist everyone (if you use whitelisting).
The mod also includes a "FULL SERVER" backup option, which can be configured to run when you enable maintenance mode, or you can perform a backup manually. The backup includes mods, server config, worlds, well, basically everything in your server folder.