Cards can belong to any of 4 Domains in the game:

Mountain Folk

Dark Legion

Sylvan

Elemental

Some cards don't belong to any Domain and are called Unaligned .

During a battle each player can gain levels in the various Domains. You can gain 3 levels in each domain. The higher the level you gain in a particular domain the more powerful cards you will be able to play from that domain. There are many ways to gain levels in a particular domain. The most common way is to build buildings that give you a level in that domain.

This Altar Of Ix Gives you a level of Dark Legion Domain.

Unaligned Cards can be played if you have any type of domain that adds up to the apropriate level.

For example this Unaligned Desolation Totem can be played once you have 2 of any combination of domains. So two Sylvan or a Dark Legion and an Elemental, etc.

You can see how much domain you have in your player status.