Hotspot network generates on the device an own network, independent from the local home network.
It’s needed for the first config setup of a new unit, or in case of the HOTSPOT FALLBACK option, to reach the unit when the local wifi network falls down due to local network troubles.
The default hotspot password is volumio2. You can set your own password in Settings → Network, in the hotspot section.
If your device is connected to the router with an Ethernet cable, you don't need the hotspot to control it: open Volumio from the app or from a browser on the same network, using the device name followed by .local or its IP address.