The interviewer wants you to describe your personality to ensure that you will be a positive and beneficial addition to their team. If you aren't sure how to express your personality, start by writing down some descriptor words that people have said about you in the past. These personality descriptors could come from co-workers, your leaders, or even your classmates.
Next, visit the company website, read their job posting, and see if you can uncover a few descriptive words that the company seems to repeat. Incorporate those particular descriptors if they apply to you. Remember - your answer needs to be honest and genuine so only use this approach if it truly resonates with who you are. Avoid giving generic, run-of-the-mill answers like 'team-player' or 'hard-working.'