This is another fairly common interview question. It is one that can be easily misinterpreted and as a result many job applicants have a tendency to venture into territory that is too personal. The interviewer is looking to connect an accomplishment that you are proud of to your professional life. As a result, you should make sure you do not talk about something that relates to your personal life. The only exception to this would be if you can connect a personal accomplishment to traits that would transfer easily to the workplace. For example, suppose that you finally managed to learn how to sail. This is something that might require dedication, concentration and commitment. All of these are admirable qualities that would be welcomed in any work environment.
What you do not want to do is to focus on some personal aspect in your life that does not correlate to your professional life. For example, you may very well be proud of your children and how well they have turned out but it can be difficult to correlate this to the workplace.
Due to the fact that this is a question which you may likely encounter when in an interview, you should make sure that you give it appropriate thought so that you will be fully prepared to answer it. Keep in mind when answering this question that you do not need to go into a lot of detail and ramble on about the particular accomplishment. Instead, it is important to make sure that your answer is instead focused on what you learned from that accomplishment and how your experience helped you to move forward with your goals.
In answering this question you might also talk about the steps that you took to help you reach the final accomplishment. This is a good way of showing that you are able to set goals, develop concrete steps for meeting those goals and then go on to achieve those goals based on a well thought out plan. This can be quite important no matter what type of job you are interviewing for and hope to be hired for.
Giving thought and consideration to this type of answer is important as you can be almost certain that you will need to provide an answer to this question at some point.