The Experience Bar shows your current experience point total and how much you need to reach next level.
This bar can also be used to track a reputation by checking "Show as Experience Bar", the amount of Artifact Power on the current Artifact weapon or the Heart of Azeroth, if equipped.
The Rest Marker shows your rest state. In rested state, you earn 200 percent of experience from kills. In normal state, you earn 100 percent of experience. Rest state does not affect quest or exploration experience.