The above is one of the classic interview questions, and a question that people inevitably hate - it's really hard to come up with a good answer, especially one that fits with whatever job it is that you're interviewing for. And yet, it's actually a question you should probably always have an answer for, whether you're in an interview or not - "I don't know" is never great.
But there's a second question that's perhaps more useful to consider, especially since it applies much more specifically, and can be asked of any ongoing task: "What does success look like?"
Sometimes, of course, that's obvious: if you're doing the washing up, success looks like a well cleaned set of dishes, possibly dried if you're showing off, and with the various work surfaces wiped down and good to go again.
Sometimes, it's a lot harder to say. But answering the question is worthwhile, as it gives a clear indication of what you're aiming for, and indeed a clear indication of what you're not aiming for. (Returning to the previous example, when doing the dishes, the aim is to get the cleaned; it's not to also go and deep clean the entire rest of the kitchen. That's absolutely not a bad thing, and may prove to be your next goal, but it's beyond the scope of what you're currently working on.)
The major benefit of stating the goal as clearly and thoroughly as you can is that it then provides shape for the intermediate steps, and probably signposts along the way. It also helps to weed out extraneous tasks, and provides a means to identify when you're off track.
Of course, the depiction of success is not a one-and-done task - at any point, it's only what success looks like from where you are now. Once you move a bit, some things that you intended may prove unrealistic (or trivial!), while you might find that your priorities shift.
And that's all good. Especially if the goal you're setting is for your life or your career - when settling that it really should be what you want, and that doesn't lock you in forever.
So, when an interviewer asks you "Where do you see yourself in five years?" what they're really asking about is The Dream - what's the goal, and in particular, what does success look like?
Oh, and incidentally, if you're interviewing for a fairly junior position, a good answer would most likely be something like "I'm just starting out in a career in <whatever>, so for the next few years I'm looking for opportunities to develop and apply my skills. In the immediate terms, I think this opportunity will allow me to learn to <something>, and we can go from there." And, if nothing else, there's absolutely no harm in saying you want to develop expertise in working in a professional team towards a common goal, and that the longer-term (5-year) goal would be to start leading a team and/or taking the lead in developing the next version of <whatever>.