Well, it is not at all a stupid question. But a very difficult one to answer. It depends on what you are trying to ask.
Your first question is, what the rules of Judo are.
Well, there is only one real rule in Judo. You alway's take care of your partner(s) and do your utmost best not to damage them in any way.
However, reading further, I think you are in fact asking what the GOAL of Judo is, since you are referring to submission.
Well, the real goal of Judo is just your development and that of your partners ( in the broadest sense ).
This is probably not at all the answer you were looking for
I guess you are mostly wondering why you don't most of the techniques you know, in competition these days. Well..... join the club....
I cannot at all explain the modern day competition rules. Every time when I think I start to understand them, they are changed again. Nowaday's competition has become more of a spectator sport ( and a very poor one ). The public is not interested in people who are rolling on the floor and try to prepare any ground techniques. That is why they only have a few seconds time in newaza. Since that is often too short to accomplish something seriously, many competitors dont even bother anymore. And since in tachiwaza many techniques are banned alltogether, about 80% of Judo is lost in competition. So dont mix up Judo with competition. They are two quite different things.