Setup breaks on your booking page
If you don't want to take bookings all day long, you have a couple options to break up your availability.
Use the break feature
From your dashboard, click Edit Settings on the booking page you want to edit.
Under Times & availability, select Repeating availability and set your general working hours.
Below that set a break covering the times you don't want to be available for booking. This could be a lunch break you take every day, or an extended break between your morning and afternoon availability. This break will be applied to every day.
Once you set the break, those times will be removed from your booking page.
Block times directly in your calendar
Any busy event on your linked calendar during your set working hours will be greyed out on your booking page. This method works well if you want to take multiple breaks in a day. If you find busy events are not being blocked on YouCanBook.me, check that you don't have units per slot enabled.
This can be used in conjunction with the break feature - Set a break for one time with the break feature, and set another break blocking the time directly on your calendar.
Use Custom Availability
Finally, you can use Custom Availability. This allows you to set very specific times when you are free within your set working hours. This is great for frequently changing availability. Details on how to set it up can be found here: Setting up Custom Availability.