Applying early means submitting your AMCAS Primary anytime in June or the first half of July.
I'm not sure if applying early actually "helps" you that much. I added my last school (a rolling one that is quite selective) to the primary in august and still got in even though I'm out of state. However, applying early is a huge psychological boost. You'll be on your 5th or 6th interview when fellow interviewees are sweating because it's their first. Furthermore, as previous posters mentioned, interviews give you an idea of where your app stands. If you're getting interviews at all the top 20 schools, you can probably cancel interviews that are less desirable. Furthermore, you can group interviews in the same city more effectively, saving a considerable amount of money (I interviewed at 3 NY schools in 3 days, which worked well since it's a long trip from California).
So... unless you're a late MCATer, submit some time in June, your future self will thank you for it.