After a concussion, which is a form of a brain injury, the brain of the person who has been injured is very vulnerable to a repeat injury. There is a time required after a concussion for the brain to heal and it's going through a number of neurological functions to try and heal itself. There is a window in which a physician will tell you not to go and do anything that might expose you to increased injury or a repeat injury.
It is so important when someone who has had a concussion to stop the activity, to get evaluated by a doctor and know exactly the time that they need to rest, be quiet and be out of the game or sport or away from the danger that can result in a second injury.
When a second concussion occurs before someone has had the opportunity to have their brain fully recover from the first one, it can have devastating consequences. It actually makes the second impact, even if it were minor, much more dangerous. This is called the secondary impact syndrome. Getting evaluated, playing it safe, getting off the bike, out of the game, or away from the activity to allow yourself to fully recover is critical.