Sure, let me explain!
The theme is an invitation to imagine mystical aspects of natural elements. This may be treating them as magical or divine entities; or interact, invoke, or even control these elemental forces; or use characters as elemental wizards, druids, shamans, elemental spirits, or ancient gods.
You could create elemental realms (like floating air castles, deep oceanic abysses, raging fire volcanoes, subterranean earth caverns, etc.), or maybe manifest them in objects controlled by the players.
Think of the theme as something that stimulates your imagination, not restricts it.