In short, magic. It’s what makes D&D different from our world. It allows the impossible to happen. Most other power sources performing seemingly magical effects, without using magic. Most of the time it requires much study and instruction. Using it without a tutor is usually a quick and messy way to commit suicide. Some people are born with an innate power, and some people are granted their powers through a contract with a powerful arcane being, but most people have studied the ebb and flow of magic and have learned how to bend it to their will with willpower, the proper incredients, and their trusty implement.