A short planned period of training harder than the body can recover from, followed by rest, so performance rebounds above where it started.
Functional overreaching is the deliberate version: one to two weeks of extra volume, performance drops, then a deload and the lifter comes back stronger. It is the mechanism behind an overload week before a deload and behind the classic three-weeks-up, one-week-down structure.
Non-functional overreaching is the same thing gone too far. Performance drops and does not rebound after a week off, and the recovery runs into weeks. Overtraining syndrome is the far end, measured in months.
The safe version is bounded in advance. Add 20 to 30 percent to weekly sets for one week, no more, and put a real deload immediately after. Never overreach without the recovery week already on the calendar.