It holds that an organism, muscle, or enzyme system stressed beyond its threshold will recover slightly beyond that level if allowed time to rebuild and repair itself. Overload work is what the phrase implies. You are stressing your body just beyond what it can handle (called demand), essentially breaking the systems down.
The systems will, however, rise to the occasion and reconstruct themselves to the level you insisted on if allowed adequate rest time. If this rest time is either too short or nonexistent, an added overload session on the same systems will only break them down further.
