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. Apprentice Wizard, or has an air frame, e.g. Castle Servants. Earth, fire, and water spells are determined in the same manner. If a spell requires multiple different elemental thresholds, e.g. Purple Knight, it is each of those elements. Knowing the element of a spell is relevant for cards like Lava Salamander, 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. Ruins, it is each of those elements. Knowing the element of a site is relevant for cards like Avatar of Earth, which specifically looks for earth elemental affinity on sites.
Air is the element of knowledge and power, dabbling in forces beyond mortal control to acquire the magical means to respond to any battlefield situation. An attunement with air grants you both mobility and tactical options thanks to its ability to teleport units around the realm and to increase its available spells.
Earth is the element of enduring strength, defined by its focus on fielding and enhancing martial forces throughout the realm. An attunement with earth grants you the ability to affect your atlas and minions that can dominate the battle through teamwork and sheer fighting power.
Fire is the element of desire and destruction, willing to do whatever it takes to secure victory in the realm. An attunement with fire grants you an array of explosive spells and abilities that can create bursts of speed or burn friend and foe alike in great gouts of flame.
Water is the element of charm and trickery, ready to adapt and disrupt the strategies of the enemy as it manipulates all. An attunement with water grants you a powerful homefield advantage that will slowly engulf the realm and the ability to position the enemy right where you want them.