The timeline shows all events stored in a Gedcom file. An event is a special Gedcom-property like BIRTh, DEATh, RESIdence or GRADuation. This view puts all events on a scale starting before the first event and ending after the last event. The events are put into a layout from left to right (and top to bottom). Scroll around to inspect the events that happened around a specific time.
The toolbar at the lower/right offers additional functionality: