Under Shop settings, Booking rules, Times and notice has three choices. Times shown every sets the steps your booking page offers: 5, 10, 15, 20, 30 or 60 minutes. Least notice is how close to an appointment someone can book online: none, 1, 2, 4 or 12 hours, a day or two days. Book up to is how far ahead: 7, 14, 30, 60, 90, 180 or 365 days.
For a gap on purpose, give the service a Clean-up after in its editor under What you sell. The time is held in the diary after every booking of it and is never sold to the customer: 45 minutes of service plus 15 of clean-up takes an hour. Without one, times are counted from the end of the previous appointment rather than from a fixed grid, so a 45-minute service does not leave 15 minutes that nobody can book.
Customers on your booking page only see times that are free, and nobody can be booked twice at once. These rules are for the booking page: when you add a booking yourself in the diary you are the authority, so it ignores your hours and notice, though it still will not put one person in two places at the same time.