The service interval on your public booking page is the length of the service by default. This ensure you can take the maximum amount of bookings per day.
You might have a service with a duration of 4 hours over working hours extending from 8am until 5pm. By default, Bookem would show slots available at 8am and 12pm, meaning you'd always have space for 2 appointments in a day. If clients were allowed to book at any time, someone could make a booking at, say, 11am - 3pm, and no further slots would be available to be booked on that day. This is because there isn't a 4 hour slot available anymore on that day: 8am - 11am is 3 hours, while 3pm - 5pm is 2 hours.
You might have some services with durations equal to 45 minutes but mostly get booked for service lengths spanning 1 hour. For this reason, you might prefer to have services always start on the hour, as opposed to sometimes 15, 30 or 45 minutes past the hour. See below:
To override this default behaviour, go to the Service Settings page and click on the service you'd like to change and click on Show More.
Then under Available every, change to a Custom duration of 1 hour.
This is how the public booking page will look now for the same service (after the page has been refreshed):