Im going to assume that you are applying to an undergraduate university in the United States.
One big thing that they want to hear is your voice. They want to get a sense of who you really are. Dont just rattle off your gpa and extracirriculars, they already know this stuff from your application. Try to get in something that makes you feel real rather just some name on a paper. One big thing that I was always taught was this "show vs tell" mentality. You do not simply want to your essay to tell them what your life is like, you want to show them. For instance, as opposed to "I have always been interested in outerspace. Because of this, I like to look at the stars at night" you want something like "As I laid on my lawn, looking into the endless abyss that is outerspace I could not help but wonder..."