This question is about your long-term career goals.
Realistically, it's hard for anyone to predict exactly what they will be doing in five years (let alone 10 or 15, which some interviewers will ask about).
What the employer really wants to know is that you have goals and that the position at hand is a good fit for your long-term planning.
HR managers want to confirm that you'll stick around if they hire you and that you see a career path with the company.
It's best to keep your answer fairly general, especially if you don't know much about the typical career path within the organization.
The key here is to stress your interest in a long-term career at the company. This is particularly important if you have any job hopping (avoid this at all cost) or short job tenures (less than a year) on your resume.
“My goal right now is to find a position at a company where I can continue to grow and take on new challenges over time. Ultimately, I'd like to take on more management responsibilities and get involved in product strategy. But most importantly, I want to work for an organization where I can build a career.”