Let's imagine that a chess game is an exam of 40 questions. I use the number 40 because that is approximately the average number of moves in an average chess game, grandmaster draws not counting. Then let's imagine that some time before the exam you are given all of the possible questions in advance. Let's say there are 1000 possible questions from which the 40 exam questions will be drawn. I picked the number 1000 out of thin air, so if you want to use another number, I won't argue with you. Those 1000 questions are equivalent to the number of opening variations, middlegame plans, and endgame themes in the arsenal of an average club player.
Your task as the taker of the exam -- its importance depends on your personal situation -- is to research as many of those 1000 questions as you can before taking the exam. You can consult books, talk to friends, practice working out examples, and even take trial exams. The only limitations are the time and other resources you have available. The more time and resources you have, the more questions you will master during your preparation. The other exam takers have also been given the 1000 questions, and your score on the exam will be relative to the others.
Chess960 is like taking the same exam without any foreknowledge of the questions. You have some idea about the types of questions you might be given, endgame themes for example, but until you open the exam you don't know with which specific questions you will be confonted. The other exam takers are in the same situation and the favorite to score highest is the person with a better grasp of the underlying subject material.
The difference between an exam where you know the questions in advance and one where you don't is equivalent to the difference between a chess game where you know the start position in advance and one where you don't. Knowing the start position allows you to discover the most effective opening variations and to explore the typical middlegame positions that arise from those variations. That's what chess players have been doing for hundreds of years with a single start position, which made for a captivating pastime until the coming of computer chess. Now it has become drudge work, memorizing scads of variations as the computer presents them. Thanks to Fischer, there's another way to approach the magnificent game of chess.