The interviewer wants to hear the reason you are leaving your current job and applying for this position. Be sure that you don't ever say anything negative about a supervisor or coworkers when answering this question, as it is in poor taste to talk bad about others. Instead, focus on the want for a greater challenge or are looking for career advancement. Highlight your compatibility with the company. perhaps your values are closely aligned. If you do not have a job you are leaving, you can explain what you currently do and why you applied to this position.