Answers to this question go one of two basic ways. Candidates try to show their incredible ambition (because that's what they think you want) by providing an extremely optimistic answer: "I want your job!" Or they try to show their humility (because that's what they think you want) by providing a meek, self-deprecating answer: "There are so many talented people here. I just want to do a great job and see where my talents take me."
This is a common one at startups (and one of our personal favorites here at The Muse). Hiring managers want to know that you not only have some background on the company, but that you're able to think critically about it and come to the table with new ideas. So, come with new ideas! What new features would you love to see? How could the company increase conversions? How could customer service be improved? You don’t need to have the company’s four-year strategy figured out, but do share your thoughts, and more importantly, show how your interests and expertise would lend themselves to the job.
Any candidate can read and regurgitate the company’s “About” page. So, when interviewers ask this, they aren't necessarily trying to gauge whether you understand the mission—they want to know whether you care about it. Start with one line that shows you understand the company's goals, using a couple key words and phrases from the website, but then go on to make it personal. Say, “I’m personally drawn to this mission because…” or “I really believe in this approach because…” and share a personal example or two.