MyVolumio account is your own account where streamer settings, streaming services, favorite music and playlists are collected.
MyVolumio account could be:
FREE (with no pro features such music from streaming services)
VIRTUOSO (basic pro plan, you can use this account only on 1 device per time)
PREMIUM (most complete plan, you can use this account on up to 6 devices simultaneously)
You can use Volumio also without account, but with less features, reduced to the local files playback (no favorites, no playlists and no settings saved on the cloud) and webradios playback.