Down to the nitty-gritty. In other words "Tell me why you want the job." The way the question is phrased, however, it means rather more than that. The kind of things which might attract you to the public sector and the Civil Service in particular include:

Working alongside ministers

Being involved in contemporary issues (often featured in the news)

Being at the heart of policy formulation

Working in the context of a not-for-profit organization (bearing in mind that this does not mean there are no financial considerations!)

Having a positive role and being relied upon to work to a high level

Team working where your "team" is the rest of the Civil Service and hence a very large and significant organization

Making a contribution to a service that really matters to people and has an impact upon them