It’s what some of the early 20th Century New Thought writers called ‘the thinking substance’. So in essence, energy is all-permeating throughout the Universe. I like to think of this as the medium through which the elements express themselves.
Each has certain distinct qualities attached to it. The four are Fire, Water, Air, and Earth. There are four elements all incorporated and overseen by a neutral fifth: spirit, or as I like to think of it, energy. Some witches connect the elements to certain deities gods or goddesses. Correspondences are things that can be connected to, explained by, or enhanced by the elements. Witches respect and use the elements as a framework and basis for magickal work and correspondences.
They understand that the elements are essential, helpful, and also very destructive. While most people give little thought to these fundamental requirements for life, witches are in tune with them.