The world is inhabited by small creatures that live alongside humans. These creatures are known as Pokémon, or Pocket Monsters, and there are hundreds of different species that live throughout the land in caves, forests, and bodies of water. Each Pokémon has a form of elemental 'type', and certain humans train hard to become Pokémon trainers, by capturing and battling these creatures, which becomes easier when you understand the type matchups, meaning what each type is strong and weak against.
Izuku Midoriya is a normal, average looking 15 year old boy, living in a not so normal world. In this story, eighty percent of the worlds population are now born with superpowers, known as "quirks".
The world is split into two realms, the first being the material world in which Humans live, known as Assiah, with the second realm being the realm of demons, known as Gehenna and ruled by none other than Satan himself. Exorcists are the ones who stand between the two realms, protecting Assiah from being overrun by the demons that try to inhabit it.