A concussion is a traumatic brain injury that affects your brain function. Effects are usually temporary but can include headaches and problems with concentration, memory, balance and coordination.
Concussions are usually caused by a blow to the head. Violently shaking the head and upper body also can cause concussions.
Some concussions cause you to lose consciousness, but most do not. It's possible to have a concussion and not realize it
Concussions are particularly common if you play a contact sport, such as football. Most people usually recover fully after a concussion.
Concussion care at Mayo Clinic