A concussion is a mild traumatic brain injury caused by the brain whipping inside the skull, which causes it to not function properly. Symptoms include headaches, dizziness, nausea, and behavioral changes. While usually temporary, concussions can have long-term effects if not properly diagnosed and treated. It is important to see a doctor after a concussion to rest the brain and prevent further injury that could lead to conditions like second impact syndrome or post-concussion syndrome.