The interviewer wants to understand your perspective of yourself. Are you good at assessing your weaknesses? This is a great way for you to demonstrate to the interviewer your reflection of yourself. This is also another way to make yourself stand apart from the other candidates.

Example:
"My weakness is that I have a Type A personality which stresses me out. I like for things to be a certain way and if a co-worker that I am working with is a bit too laid back, then I get annoyed. I am actively working on being patient with others as not everyone is going to be like me."