To add a one-off block to a resource's calendar
Select the time interval in the calendar
Click Add blocked time
Change the duration
Confirm the date and time and click Next
Add a Label (this will be displayed on the calendar)
Add notes to this break if required
If you want to add regular breaks e.g. at the same time every Monday. You can add recurring breaks in the working hours of the resource. You'll find a guide to this here.