The XP Bar is a meter which shows your level progress. In the blue star on the left side your current level is shown. In the bar itself your current Experience Points are shown. The meter will fill up with blue if you gain more Experience Points. Once you have reach a full XP Bar your level will be increased by one and the bar will be empty again.
If you put your mouse over the XP Bar a popup will be shown which tells you how much Experience Points you need until the next level.