The interviewer would like to know how soon you would be available to start the position if they made you an offer. If you're currently employed, it's okay to share that you'd like to give your employer a notice period. If you're flexible and can work some hours outside of your current job schedule and notice period, share that with the interviewer. It's appropriate for you to ask how soon they are looking for someone to start, that way you have a better understanding of their expectations and can see if you can come up with a solution that works for you and the organization.