The notice period sets out how much time you require before a new booking. E.g. if your notice period is set to 1 hour, someone can make a 4pm booking up until 3pm, but no later.
The notice period can either be set for the account as a whole by going to Booking tools > Online booking settings and adjusting the Booking policies
Or you can make the notice period service specific by inputting the notice period within the service settings. You can edit a service by going to Your business > Services and clicking on the pencil icon to edit.