Events are another name for the time slot when an activity or resource is available. Both Activities and Resources require Events to be added before an activity can be booked.
An Event could be a fixed time when an activity takes place (e.g. a restaurant sitting), or a longer duration, during which an activity can take place (e.g. the opening hours for the spa).
If an Activity has a fixed time, then the Event must also be the same length of time, or longer.