Recovery is as important as any workout. Given that the goal of training is improvement and this is achieved through overload (read tearing down muscle), gains can only be had by properly building our bodies back up through rest and proper nutrition.
While recovery seems like a simple concept it becomes much nuanced in practice once training plans, events, social pressures, etc… are factored in. Compromises invariably result. Extreme cases are easily identifiable but the grey areas are much less so. On one side if recovery cannot be had it would be best to skip that workout altogether or risk training be set back. On the other side too much recovery will lead to seeing non-optimal gains. This concept is expressed graphically below.
All of us have a different recovery profile. As our general and sport specific fitness increases our respective recovery profile changes. By experimentation and taking cues from our bodies we can discover our own recovery profile.
Of special interest is the part of the curve where gains turn negative. In blunt terms we can expect our fitness to decline if we engage in demanding physical activity and follow it up with a night of drinking and partying, working, studying for exams etc... Someone also risks negative gains by overtraining if workouts are spaced too closely. Less than optimal gains are tolerable for most everyone and often result from informed compromises. Negative gains and a loss of fitness are frustrating for everyone.