Active Rest

Whether you are talking about being active between workout days or performing less intense exercise in between more intense workout sessions such as with high intensity interval training (HIIT), active rest is key to giving your body the recovery it needs so your workouts are effectively intense. 

*Rest days in between days of resistance (strength) training workouts is important because it allows your muscles enough time to recover, repair, and grow stronger.  Rest however doesn't mean couch time.  Active rest would be a less intense activity such as a walk (even a jog), bike ride, yoga, etc.  Get the idea?

*A lower intensity exercise in between HIIT gives your body enough recovery while preventing your heart rate from dropping rapidly.  Without the active recovery period you would not be able to maintain the intensity required in HIIT to make it effective.