As a former HR manager at a staffing company, I would always ask this question. There was never a correct or expected response. I used it so that the interviewee could use their own words to tell me who they are, what they can do, and what they want to do.
For instance, if a person came in looking for work and their response to this question was to tell me their life story or to say, "I don't know, I just want a job". The chances for that person to be successful in getting placed or staying placed in a job would be low.
Whereas, the one that comes in and says "I started out working with my dad on construction sites, and I have really enjoyed that and would like to continue to do construction. I have multiple heavy equipment operation certifications, along with training in multiple areas of a standard site. My dream is to become a foreman or even own my own company one day." This person would be more successful in getting and staying employed.
The trick is to remember that it is not about what you answer, it's how you answer. How do you "sell" yourself. How do you present yourself in a way to makes the company/institution want you?
Now, your answer doesn't need to be super detailed or long. But detailed enough and exact that you relay all the needed information.
Main points you will want to address:
Professional background
Abilities and achievements
Goals
Why you're excited about the opportunity
Have an answer before you go. This way, you're not stuck sitting there thinking. Being prepared is always best, it always helps build confidence as well.