Answer:
It is the chain reaction process that is used in reactors and nuclear weapons to generate a large number of fissions. In a reactor, the propagation of fissions takes place in a controlled manner, in a nuclear weapon in an uncontrolled, explosive manner.
The products of nuclear fission are usually accompanied by two to three secondary, free neutrons which can be absorbed later by other fissile nuclei. These absorptions in turn lead to further fissions, then to the production of other neutrons etc ..., a process commonly called chain reaction.