Sodium, potassium and magnesium. Largely. Varying concentration levels inside and outside our cells create positive and negative gradients which stimulate cellular reactions. Basically they have to do with our body's electrical system and are crucial for creating action potentials which start and end nerve impulses, make our muscles relax contract, operate our hearts, allow us to see and a ton of other important shit. It's the chemical and atomic level of our bodily functioning, very interesting stuff, yo.