The default functionality of the customer quote form is to have the customer enter any start time and end time they want. Here, you can choose instead for the customer to choose specific timeslots from a drop down.
You can also set up Timeslots for just certain items. The customer will chose a regular start and end time for the whole event, but then the specific rental item will prompt the customer to choose a specific timeslot for that item. To add timeslots to a specific rental, search for it under "item name", press +add and it will allow the timselot to show for that item.