These are neutrons that have reached the thermal equilibrium according to the Maxwell-Boltzmann distribution after having bounced around many times without undergoing neutron capture.
Good fissile material is material that is able to absorb thermal neutrons and continue the reaction, because that's the type of neutron you end up getting the most of.
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Neutron temperature is a concept used to describe the average energy of neutrons in a system, typically in the context of nuclear reactors or neutron scattering experiments. It is similar to the thermal temperature used in classical thermodynamics, but it specifically relates to the kinetic energy of neutrons. In essence, neutrons, like other particles, can be assigned a temperature based on their energy distribution. When neutrons are in a thermal equilibrium state, their energies can be characterized by a temperature value.