Understanding the answer to this question starts with learning what happens when you suffer a concussion.
The brain is a soft organ, cushioned to protect against day-to-day bumps and shakes. When an impact is especially violent, however, the brain can collide against the solid walls of the skull and sustain damage. This is a concussion.
The symptoms associated with a concussion include headaches, dizziness, sensitivity to bright lights and vision problems, among others. Essentially, a concussion is an injury to the brain that needs time to heal properly. For it to do so, you need to rest. You also need to avoid activities that put strain on the brain or affect the way it functions.