Metahumans are enhanced beings living on Earth who possess unique abilities that most average humans don't. The exact cause of Metahumans is unknown though it's implied that the Metahuman gene lies dormant within every human.
In some cases, a person can be born a Metahuman while in others, the Metahuman starts out as an ordinary human who ends up experiencing life-threatening situations resulting in their Metahuman genes awakening.