It is based on a comic book series of the same name. 
The "X-Men" are people who have undergone the next phase of evolution, each one possessing a unique newly acquired ability.
The X-Men series follows their stories, with 2 sides of X-Men often battling each other over their worldview. With one team who wants to co-exist peacefully with humans, and the other who believe that humans and mutants should never co-exist.
X-Men mainly uses fantasy elements relating to transhumanism, the X-Men themselves and their enhanced abilities link closely to humans who want to be better than themselves. They are somewhat transhuman in their own right.