This is a very straightforward question. While there is no way to provide a generic answer to this question and you will need to provide an answer that is honest, it does give you the opportunity to really play up your skills and experience and what you can do for the company if they should decide to hire you.
The biggest problem with this question occurs when you were never promoted in your prior position and you feel the need to explain that. If that is the case you must be extremely careful in how you respond to this question. Under no circumstances should you talk in a negative fashion about your prior supervisors or lay the blame for a lack of promotion at your old job. This will provide the prospective employer with a bad impression and make it seem as though you have a habit of bad mouthing your supervisors.
Instead, if you were not promoted during your prior position you can simply state that there was not an opportunity for advancement, which is something that you look forward to if you should be hired for the position that you are interviewing for. At that point you should then go on to correlate how your skills and experience relate to the required qualifications for that job.
Of course, if you were promoted in your last job, this is a golden opportunity because it allows you to discuss your skills and experience as well as achievements that you may not have been able to fit on your resume. This is an opportunity that you should never pass up but should instead make the best use of in order to make the best impression possible during the interview.
Remember that when you respond to this type of question you should never give any information that is not absolutely true. With this form of question it is too easy for the employer to check up on your answer when checking references and find out if you were indeed telling the truth. The best policy is to always tell the truth and try to put it in the best light possible in order to highlight your skills and experience and how they relate to the requirements of the job that you are interviewing for.