1) A good site supervisor PREPARES you for the tasks that you will be responsible for during your internship -- This means the supervisor provides you with the necessary knowledge that you will need and is available for answering questions.
2) A good site supervisor MENTORS you by being available on site when completing tasks during your internship. The supervisors introduces you to people and helps you gain familiarity with the organization
3) A good site supervisor gives you OPPORTUNITY FOR ACQUIRING SKILLS.
The site supervisor should insure that you are not required to simply do the paperwork that s/he does not have time to do. You are not an assistant to the site supervisor. Rather you are a trainee. As such, the site supervisor should provide you the chance for gaining needed skills.
Be aware that the site supervisor will be requested to write an evaluation of you after the completion of your internship.