From the introduction:
In the world of Psyren, there are people known among themselves as Psychers or simply Psychics. They derive their abilities through overclocking their own brains to accomplish tangible effects in the world around them, enhance their own bodies, and any number of more esoteric effects.
Because of the means by which they do this, overuse of psi without a rest will lead first to a headache, then to a fever and a nosebleed, and finally to death.
To be a psychic, a certain natural safety barrier that is built into your brain must be removed. Some psychics are simply born without it. Some have an accident of some kind in which they receive a concussion which just happens to remove it.
Others breathe the strange atmosphere of Psyren, the wasteland that will be the future of the world, and lose it that way. Psi abilities are also much more powerful in Psyren than in the present, due to the oddly contaminated air being a better conductor for psi energies.
One way or another, you are or will be one of them.