Literally translated, pro per means for one's self. The term is often seen in the courts when a defendant chooses to represent themselves. The United States allows for any defendant to be able to choose to deny legal counsel and hear the facts of the case as the defense and the defendant.
A judge can rule against this right in a criminal case where the judge feels the person needs to be examined as competent. Being competent to represent yourself is the only precondition. This does not mean you need any legal experience. It only certifies that you are mentally competent. Pro per can exist in any type of law: criminal, civil, tort, etc.