Although an average of two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction - some escape from the reaction vessel - some interact with non-fissile atomic nuclei. If the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.
The point where the chain reaction can become self-sustaining is referred to as critical mass.
The critical mass is the smallest mass of fissile material required for a nuclear chain reaction to be maintained. The output from a critical mass is steady because there is an equilibrium between the number of neutrons produced and those causing fissions.
The critical mass of a fissile (able to undergo fission) material depends upon:
Surrounding the fissionable material with a suitable neutron "reflector" (beryllium) - the loss of neutrons can reduced and the critical mass can be reduced. (By using a neutron reflector, only about 11 pounds (5 kilograms) of nearly pure or weapon grade plutonium 239 or about 33 pounds (15 kilograms) uranium 235 is needed to achieve critical mass).
A subcritical mass is a mass of fissile material that does not have the ability to sustain a fission reaction. The number of neutrons produced by a subcritical mass will exponentially decrease. This usually happens very rapidly as they escape from the fissile material faster than they can be produced by fission reactions.
A supercritical mass is one where the rate of fission is increasing. The material may settle into equilibrium (i. e. become critical again) at a higher temperature or power level.