It means that champions in your team have to be from the same faction.

The factions are as below:

High Elves, Banner Lords, Barbarians and Sacred Order

Orcs, Orgyn, Lizardmen and Skinwalkers

Dwarves

You are supposed to use champions from either of the three groups above

No, he has to use champions from one of 13 factions.

More importantly- WHY DON'T THE GAME DEVS DEFINE THIS IN THE INFO SECTION!!?? SO FRUSTRATING!!

You forgot Demonspawn, Undead Hordes, Dark Elves, and Knight Revenant.

Each line of factions also has a name, which may appear in the special rules for events and tournaments.

Telerians, Gaellen Pact, The Corrupted, Nyresan Union. The last one only contains a single faction, Dwarves, so far.

I think the rule about the mono-faction teams is about a single faction, not a group of factions, though.

I honestly thought it was clear... Mono being ONE, which means you can only use 1 faction to make up for the toons in your classic Arena team, which faction is that? Any faction as long as the whole team is comprised of that one single Faction that you choose.