In the next few days, I'll be reviewing all the candidates and will start scheduling second interviews. What's your availability like over the next few days?

They are likely asking this question of every interviewee. It could be a test to gauge how presumptive you'd be in your translation of their intensions in asking. Remain professional and objective. Considering how competitive this opportunity is, being completely available and ready for the next stage when it is presented reflects that you have planned well for this opportunity. A willingness to be flexible, while conveying hopeful enthusiasm, is the tone you'll want to strike.