Exercise has many positive health effects. Going from a sedentary lifestyle to an active one will lower your blood pressure, decrease your risk for certain diseases and make you feel better about yourself. Some people might take this a step further and justify over exercising by thinking that if a little bit of exercise is good, a lot must be even better.
Too much exercise can be unhealthy and will set you back. Overtraining happens when you exercise too frequently. When you don't allow your body to repair that damage that's caused by exercise, you'll start to get overly sore, fatigued and eventually injured.
Exercising causes small tears in the muscle fibers. When you rest, your body finds these tears and repairs the damage. If you exercise too much, your body will never have the time to fix the damage and your performance will suffer.
Exercising everyday isn't necessarily bad if you schedule your workouts properly. Don't exercise the same muscle group more than once every three days. You should also avoid exercising any muscle group that's still sore from a previous workout.