This would be a typical interview question of any internship interview. The answer would / should be unique to your situation. Hopefully you know what you want to gain, but now you simply have to articulate it w/ some passion in the interview.

Typically interns want to gain experience and learn new skills in a particular industry. Focus on a couple specific skills you hope to learn from your experience and answer the question with the list of those experiences.

If you're hoping the internship turns into a full-time position, you can mention that, but do not make that your answer to this question. An answer to a question about a full-time opportunity should be positioned as "while I am an intern here and I find the work environment a good-fit for me and you're happy with my productivity and performance would there be an opportunity full-time employment?"

This piece was cogent, well-wirtten, and pithy.