This is a tough question to answer without dipping into platitudes. Try sharing leadership examples instead. Say, "The best way for me to answer that is to give you a few examples of leadership challenges I've faced," and then share situations where you dealt with a problem, motivated a team, worked through a crisis. Explain what you did and that will give the interviewer a great sense of how you lead.
What Interview Questions Are Illegal?
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.