Point speech-to-text, text-to-speech, and translation at servers you run yourself, instead of the public defaults. List more than one server and the device tries them in order, moving to the next one if a server does not answer — that is the redundancy.
Each list only takes effect when the device is configured to use that engine's server plugin (for example ovos-stt-plugin-server). Set the plugin on the Settings page first.
Leaving a list empty is fine: the matching server plugin then falls back to its own built-in public servers.