Generally a recovery ride is a good idea after doing a "breakthrough" ride - one where you've ridden much longer and/or harder than you normally ride. Your body will need some additional time to recover and build itself up after such rides, so it's good to take it easy the day after to let the recovery process yield maximum benefit. Riding at an easy pace for a short distance (anywhere from a few miles to 10 miles, but there is no "standard" length) encourages blood flow in your muscles, helps to flush out toxins, avoid stiffness, etc.