Override Hours for Specific Days is the section that overrides the Regular Weekly Hours. This is great if you have special events or things that come up in every day life and would like to have special times on specific days.
If you have different hours entered for a special event such as Monday, April 14th in the Override Hours, those will override your regular Monday hours for only April 14th.
You can update or add any available hours on specific dates by selecting "Add date overrides". Adding a date override builds new availability for that specific date instead of your normal schedule.