Definition
The bounce above the previous baseline that follows a training stress and enough recovery, which is the adaptation the next session builds on.
The curve: stress, a dip in performance, recovery back to baseline, an overshoot above it, and then a slow return to baseline if nothing else happens. Train during the overshoot and the baseline moves up.
The overshoot arrives 24 to 72 hours after most sessions, later after heavy eccentric or high-volume work. Train too soon, repeatedly, and fatigue accumulates; wait too long and the gain is lost.
The model is a simplification (the fitness-fatigue model describes it better), but it is the clearest picture to give a client who asks why they cannot train chest every day.