A concussion is an injury that changes how the cells in the brain normally work. A concussion is caused by a blow to the head or body that causes the brain to move rapidly inside the skull. Even a "ding" "getting your bell rung," or what seems to be a mild bump or blow to the head can be serious. Concussions can also result from a fall or from players colliding with each other or with obstacles, such as goalpost.
The potential for concussions is greatest in athletic environments where collisions are common. Concussions can occur, however, in any organized or unorganized sport or recreational activity. As many as 3.8 million sports and recreation related concussions occur in the United States each year.
Borrowed from Center for Disease Control and Prevention