Sorcery: Contested Realm is built around the four classical elements: Air, Earth, Fire, and Water. To cast spells, you'll generally need to have elemental affinity.
A spell is an air spell if it requires one or more air threshold, e.g. , or has an air frame, e.g. . Earth, fire, and water spells are determined in the same manner. If a spell requires multiple different elemental thresholds, e.g. , it is each of those elements. Knowing the element of a spell is relevant for cards like , which can only cast fire spells and takes no damage from fire spells.
A site is an air site if it provides one or more air affinity. Earth, fire, and water sites are determined in the same manner. If a site provides multiple different elemental affinities, e.g. , it is each of those elements. Knowing the element of a site is relevant for cards like , which specifically looks for earth elemental affinity on sites.