Hello! I love this question!
To begin... what is nuclear fission? Nuclear fission is a "system" in which a big nucleus separates into 2 smaller nuclei. As this happens, it also releases neutrons and energy.
So... you start with a neutron. Then you have a nucleus, which splits into smaller nuclei and a few neutrons. These neutrons then split off and hit more nuclei and this just continues on to create more and more.
Now... when looking at what is created from fission... after the whole process is complete, you are left with fragmented atoms.