Final answer:
A jagged or slanted line on a timeline usually means that a certain period has been omitted to conserve space or focus on key events.
Step-by-step explanation:
A jagged or slanted line on a timeline typically represents that a certain period of time has been omitted from the timeline. This is usually done to conserve space or to avoid detailing a period where there are no events relevant to the topic being represented by the timeline. It is not commonly used to indicate that nothing of historical importance happened or that the history is lost or unknown. Instead, it is a design choice to make the timeline more readable and focused on the key events or periods of interest.