Each bar represents one task or milestone. The bars are laid out in chronological order.
You can use Preferences from the Edit menu to specify which bars you want showing and whether you want slack time displayed.
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The planned bar (the wide one) shows the elapsed time planned for the task.
The actual bar (the narrow one) shows the elapsed time for the task based on any actual dates you have provided. If you have not added any actual dates to the project, the actual bar has the same size and position as the planned bar.