What is the meaning of oil-water separation method in painting

and frying the same principle, water and oil are incompatible, will fry the pan, painting is the first with oily drawing materials, such as oily colored pencils crayons markers, etc., and then add water-based drawing materials such as watercolor watercolor and gouache and so on, there are oily places where the water will be naturally separated, can not be covered, this technique is the separation of oil and water technique