Espers are individuals with abnormal or supernatural abilities that usually give them an edge or an advantage over normal Human beings, such as the ability to Teleport or of Psychokinesis (the ability to move objects with one's mind. In Rou's world, Espers are somewhat rare, and the age they awaken their power is unknown.
They are the equivalent of Mages in Fantasy worlds in more advanced or Sci-fi universes.
For instance, Rou possesses the ability of Absorption.
It is entirely possible that individuals from Rou's reincarnation world possess similar abilities not related to magic, although it is difficult to differenciate them.