I recently answered this question for a German magazine. It's an issue that is seldom discussed and often taken for granted–recovery.
Question: Why are recovery days so essential? What happens to the body on a recovery day? What’s the best way to plan a recovery day?
Answer: I tell athletes that the hard training days only create the potential for fitness. They don’t result in fitness improvements unless there is rest. For it’s during short-term rest that the body adapts to the stresses of exercise. Muscle strength and endurance improves. The heart’s stroke volume (amount of blood pumped per beat) increases. Capillary beds in muscles grow allowing the heart to deliver more oxygen. Aerobic enzymes increase. Blood volume increases further enhancing oxygen delivery. Glycogen stores are restocked allowing for harder workouts in the following days. And these are only some of the physical changes that result from recovery.
Recovery days come in two forms: days of complete rest ("passive" recovery) and days with light exercise ("active" recovery). Passive recovery is generally best for novices. If they take the day off from exercise the day after a workout they will improve greatly. For the pure novice any form of training may very well be too stressful. As fitness improves, the recovery days are better spent doing some very light exercise. For the novice this could be light cross training in a sport such as swimming or cycling. Novice runners should never run on a recovery day. It's simply too stressful even for somewhat advanced novices.
The advanced, experienced athlete is best advised to train lightly on a recovery day as this maintains some of the most basic gains made in previous, harder sessions, especially economy of movement and aerobic endurance. Given the advanced athlete's high level of fitness, such a light training session is not stressful. But it must be easy. Making these sessions too hard is the most common mistake in training at this level.
Regardless of one’s level of experience or fitness, the harder the hard workouts, the easier one’s recovery days should be.