You are human, and with your existence comes infinite possibilities. Being the most flexible and adaptable race, facing constant adversity you have been born with powers to emphasize that.
Magic is innate in humans, though not all have the ability to use it. While other races rely on other sources of magic, such as runes, the magic of man is embed within them. Each human has a trace amount of power within them which can manifest itself in many ways depending on personality, bloodline, or training. Though anything can be learned with time and patience the body can only handle so much raw power. Humans have defined amounts of a resource within them that can be used as a conduit to bring out their magic and project their wills upon the physical world. This resource is deemed mana, as common to man as blood and functions similarly. Flowing throughout the human body to allow it to function and use magic normally. Mana is expended when using magic, but it never goes away. Once mana is used, it must be restored over time by breathing, oxygen refilling mana at rates that depend based on the person.
That said, some people are born with mana, but an inability to use magic naturally. Those people are either condemned to a life without magic, or seek it elsewhere. People of great faith and devoted life to the human pantheons may be graced with powers given to them by the gods. Or, they may seek a patron demon to work for, similarly to how gods work.
