Nuclear fission is the splitting of a large atomic nucleus into smaller nuclei.
The entire nucleus splits into two large fragments called 'daughter nuclei'. In addition to the 'daughter' products, two or three neutrons also explode out of the fission reaction and these can collide with other uranium nuclei to cause further fission reactions. This is known as a chain reaction.
The fast moving neutrons carry most of the energy from the reaction with them (99%) but before the neutrons can collide with fresh uranium nuclei, they need to be slowed down.