Overtraining for long periods of time is bad. Short-term, planned overtraining, however, can be a massively powerful tool.
Functional overreaching is essentially short-term overtraining where you have a goal of digging yourself into a recovery ditch. You intentionally push your training past your body’s ability to recover before backing off, super-compensating, and jumping out of that recovery hole to new levels of strength and muscle. Doing so allows you to benefit from the harder training as your body gets a chance to recover.