The Scheduling page (in the sidebar) is where you manage time clients can't book, on top of your regular weekly hours. Your actual open slots are always your working hours minus other bookings minus these blocks.
Use Time off for whole days — vacations, personal days, holidays. Click Add time off, pick a start and (optionally) end date, and an optional label. Remove one with the trash icon.
Use Recurring blocks for a time reserved every week, like a daily lunch break. Pick which weekdays it applies to, a start and end time, and an optional label. Recurring blocks are hard windows — no buffer time is applied on top of them the way it is for other bookings.