In fission, a is one where a neutron imbeds itself in the nucleus of a u-235 atom causing it to break apart. after destructing, more neutrons are created. these neutrons can carry on the cycle by finding more u-235, and, again, starting the fissioning process.

fisile atom

Explanation:

Nuclear fission is the reaction in which the nucleus of a heavy atom (U-235 atom for example), when bombarded with neutrons (or when capturing an incident neutron), becomes unstable and divides into two or more nuclei of lighter atoms, giving rise to a situation of greater stability. In this process that takes place in the atomic nucleus, neutrons, gamma rays and large amounts of energy are emitted.  

In addition, several neutrons are produced that affect other fissile nuclei,and trigger more fission reactions that in turn generate more neutrons. Effect known as chain reaction.