How do fission reactions release energy?
Some mass is converted into energy.
During fission the reactants lose some mass, i.e. the mass of the products is less than the mass of the reactants. The difference in mass is called the mass defect, Δm.
What happens to the missing mass is that it is converted into energy according to Einstein's equation from special relativity:
The form of energy output from a fission reaction is kinetic energy, the product particles move with high speed.