Developer here, thanks for watching the gameplay! To explain the basics: you can control the elements (fire, water, and earth) by "taking" an element from the environment into your hands, then throwing it somewhere else. Like taking fire from a campfire and using it to ignite a bridge, or taking grass and throwing it at a wall to create a vine to climb up.
We're super inspired by Avatar: The Last Airbender and wanted to make a stealth game where the elemental interactions were about giving you increased utility and mobility. We basically set out to make Thief/Dishonored/Mark of the Ninja, by way of Avatar.
And yes, co-op is entirely optional! The whole campaign is the same for both singleplayer and (same-screen only) co-op.
The announcement trailer gives you a good idea of how the game works and some of the possible interactions:
