A visual representation of the number of events returned by a search over a selected time range. The timeline is a type of histogram, where the range is broken up into smaller time intervals (such as seconds, minutes, hours, or days), and the count of events for each interval appears in column form.
When you use the timeline to display the results of real-time searches, the timeline represents the sliding time range window covered by the real-time search.
For more information
In the Search Manual: