How do you become an over achiever?

I recently had a job internship at GE and I dont think I did what my bosses wanted me to. At then end of the internship I had a talk with my boss and she told me that I "didnt take the extra mile." I dont quite understand what she means. What I did on a daily basis was go to work, work on my projects, then go home. Is there something that I'm missing? Coming from a professional standpoint???