We know that ‘U-235’ atom is unstable. Its nucleus is balanced with a delicate bond. If this balance is slightly disturbed, it undergoes fission. So based on this principle, the nucleus of U-235 is bombarded by slow neutrons.
The nucleus of U-235 undergoes fission releasing two or more neutrons also. When these neutrons strike other neighboring U-235 then their nuclei also undergoes fission and side by side three or more neutrons are also released. If these three neutrons further strike the nuclei of other U-235, then each will release three more neutrons. This way these nine neutrons strike with the nuclei of nine more U-235 and release 27 more neutrons. Thus, more and more neutrons strike more and more nuclei releasing more and more energy. This process of multiplication is called chain reaction. If this reaction is out of control, then there will be nuclear explosion like bomb.