What did Jobs teach all of us? Where do you begin? But, one key thing is focus -- the need to focus to succeed (on what really matters in a product, on what really matters in your business, and on what really matters in your life). And, focus means the discipline to say "No!" You can't do it all. So, whatever you do, do it well ... really well.
Here is a great video from about 15 years ago when Jobs made this fundamental point. Enjoy it. Internalize it.