A Culture is a group of peoples with similar language and customs. They may have associated Traits which Heroes and Notables of that culture will be predisposed to possess, and they together with Race strongly influence the way AI entities of the culture approach their goals - e.g. a culture valuing Cunning and Deviousness is more likely to sabotage a rival stealthily than one which is more Trusting and Honourable.
To be usable in game a Race must be able to participate in at least one culture, though typically it will have multiple. Most POIs and Nations will feature a mixture of cultures, though often with a predominant majority culture.