Complex sentence: A supercritical mass is one where there is an increasing rate of fission until natural feedback mechanisms cause the reactor to settle into equilibrium (i.e. be critical) at an elevated temperature/power level or destroy itself (disassembly is an equilibrium state).
Split into simpler sentences: A supercritical mass is one where there is an increasing rate of fission. The material may settle into equilibrium (i.e. become critical again) at an elevated temperature/power level or destroy itself (disassembly is an equilibrium state).