By default a new available slot is only generated at the end of service length. So if your service lasts for 60 minutes then a new slot will be generated every 60 minutes as long as there is availability. Thus opening hours set to 09:00-14:00 will have the following available slots: 09:00, 10:00, 11:00, 12:00, 13:00.
To offer more choices to the customer it is possible to define how often a new slot should be generated using the 'Available slot every' option on the service. You can for example set it to 30 minutes. With the same opening hours it will result in the following available slots: 09:00, 09:30, 10:00, 10:30, 11:00, 11:30, 12:00, 12:30, 13:00.
While this can be helpful for the customer please be aware of the potential problems of such a setup: you might end up with unbookable times in your calendar. If a customer books 09:30-10:30 and another 11:00-12:30, you will be unbookable between 09:00-09:30 and 10:30-11:00 as it is not long enough for a service of 60 minutes to be booked.