A timeline enables you to mark an event that occurs during a video, and enter metadata to describe that event.
Example: you have a video of a soccer match. Every time a goal is scored, you could place a marker that contains metadata describing the details of the goal such as: the player who scored it, the goalkeeper, the team, and so on.
You can apply multiple timelines to the same asset if necessary. For example: you might want a separate timeline for goals, red cards, yellow cards, player injuries, and so on.