Mysterious College decision thing.
^ More likely than not, no one on CC is a college admissions officer or of any authority to really explain exactly how admissions officers make their decisions. We just know, in general, what colleges are looking for in their potential freshman classes, and we can really only gauge what qualities competitive applicants need to have, theoretically. Carman788 explained this pretty well.
However, you mainly asked this question for advice in how to approach your high school career, I'm assuming. You can read through CC's many result threads and helpful advice posts to figure out what colleges want out of you. But my best advice to you (though like everyone else on here, I am no authority on the subject) is to be TRUE to yourself. Do the best that you can possibly do in all of your classes and take the time and effort to study hard for all your standardized tests. But join clubs that actually matter to you and only participate in activities that you find a passion for being involved in. You'll be devoted, have fun, and gain those coveted leadership positions and valuable experience that colleges like to see when making their decisions.
If you stay true to yourself, some colleges will accept you and some may not. But, ultimately, you'll be proud of what you accomplished and find that the college you do end up going to will probably be the perfect fit. That's another great perk of the whole mysterious college decisions thing.