In personal injury claims, the legal issue is the liability. You must prove that the party that injured you failed to act as a reasonable person would have acted in the same or similar circumstances: they were negligent.
In automobile accidents, that can mean someone violated a traffic law: they turned left in front of you, were speeding, ran a stoplight, were drunk driving, etc. Those kinds of things prove negligence. But negligence then has to result in an injury, whether it's financial or personal. Then, you have to prove that the injury was the direct result of the negligent action.
Proving your personal injury case can sometimes be complex, depending on the medical information that you have. This is why it is so important to have an experienced personal injury attorney on your side.