These injuries are called concussions. A common injury that results as a blow to the head, a lot of times during sport, fights, or accidents. A sudden blow to the head causes shaking of the brain inside the skull causes acute symptoms, not usually dangerous, unless they are repeated, or it is super severe. It is usually met with headaches and unconsciousness, and is usually treated with rest.
Remember though that there are other injuries that could happen, which could cause severe bleeding of the brain, and so one should not be conservative when seeking treatment.