Fission reactions are reactions where larger elements are broken into smaller elements.
Further, some fission reactions are not completely spontaneous, they are induced by a collision with a neutron. If a reaction is induced by an incoming neutron and the fission process kicks out more neutrons, then a chain reaction can occur, because the kicked out neutrons can collide with other nearby atoms and induce those to fission which kicks out more neutrons which induces more fissions, etc.