This is all Tesni's in character recollection of a now destroyed text in a summarized and paraphrased form:
To understand magic is to understand the world. To wield magic is to wield the world. Just as there are always new things to discover, your magic may grow without limit. Just as no one may grasp the whole world, your magic will always be limited. This is the truth of magic.
The world is made up of five elements. Fire, Earth, Wind, and Water are the commonly understood elements, and the fifth element is spirit. Everything that exists within the world is some combination of these five elements. A witch is a person who uses the spirit to effect change upon the other four elements. This is the essence of magic.
Spirit is the strangest element, and is the element of life, thought, and emotion. Every living thing has spirit, from the meanest weed and insect, to the most majestic animals. People, of course, have spirit, and in fact have an overflowing abundance of it. So potent is the human spirit that with effort and care it can impart itself upon others, and even things that never had a spirit at all. A cherished pet, a lovingly tended garden, or a treasured keepsake all gain some measure of spirit from people, though minuscule and quick to fade. It is through a similar mechanism that familiars were once created, before the now obvious (but unknown to Tesni) consequences were discovered and the practice abandoned.
Magic involving the spirits of beings of complex matter (that is, things made up of different elements) is difficult, dangerous, and often of surprising little use. Therefore, my unseen daughters, I will focus on magic dealing with pure elemental spirits.
Pure elemental spirits are very different than people, or animals, or even plants. Their body is not a complicated mix of elements, but purely their own element, and the age and power of spirits vary widely. They arise naturally where their element exists, and just as naturally disperse, to reform again. This all is not quite true, but it is a useful lie to begin to understand. A witch gains the power to move the element by beseeching the spirit, just as a normal person might move another by asking. This is the beginning and ending of magic, but as the difference between a simpleton getting a lone man to take a step to the side and an eloquent speaker moving multitudes towards a single purpose is plain to see, so too is the difference between a novice and experienced witch.
To put it plainly, magic is simply asking a favor. You might improve your odds by using words your audience will find pleasing, by giving gifts that they are receptive to, asking for something they were already inclined to do, or simply being the type of person they like, but in the end it's still simply making a request of another. A witch has no power, yet a witch's request can topple the world.