What is the difference between "start times" and "opening hours"?

The availability type is chosen when you create the product, and can be changed later in Settings, in the product sidebar.

Start times suit products that begin at a fixed time: a guided tour, a music lesson, a yoga class. You declare departures at 9am, 10am and 2pm, for example.

Opening hours suit offers available throughout a time range: a museum, for instance, whose visitors come in whenever they like between 9am and 6pm.

You can mix both: nothing stops you from having one product with fixed start times and another with opening hours in the same catalog.