The Away Dates feature lets you set up your staff's vacation days so they aren't available to take appointments. You'll find it under the "Away Dates" tab on the left side of Staff pages. (Click on the Settings gear at the top right, then "Staff" and the staff member's name.)
If a staff member is only away for one or two days, or only lets you know at the last minute, there's an easy, one-click way to make them unavailable right in your Calendar. Just click the icon that looks like a clock next to the staff member's name in the day view, or next to the date in the week view.
Once you've done so, you'll see that every time slot has the word "Away" in it.