A law school outline is a condensed version of the important rules of law in a course such as Contracts, Torts, or Property.
While you can buy commercial outlines, it is a better idea to form your own law school outline based on assigned reading, class notes, and any secondary sources recommended by your professor; your outline should be your primary study material when preparing for a law school exam.
Creating your own law school outline also serves as a method of studying as it will also help you remember the important rules of law as you work through the material again and rephrase things in your own words.