They're gauging your flexibility, your expectations, and if you'll be a reliable employee. They're also interested in uncovering any scheduling conflicts you may have that would conflict with their schedule and overburden the rest of your team if your attendance is a problem. It is best to be honest. If you have other priorities or demands on your time that aren't agreeable to working lots of overtime or covering other shifts, clarifying this now is better for all concerned earlier rather than later.
