XP is awarded when you complete certain tasks in the game, such as wining battles, upgrading your buildings, and progressing through one of the single-player campaigns.
XP contributes towards your Kingdom Level, which is an overall measurement of the strength and power of your Kingdom.
Player XP[ | ]
You can check your current Player XP in your Profile. For example if your profile says "Player XP 400000/406170" then 400000 XP is your current XP and 406170 XP is when you will level up next. The first number is your total XP, meaning it is needed to level up from level 1 to your current level and the latter from level 1 to the next level.
Lv 22 330916 XP
Lv 23 406170 XP
You need 406170-330916=75254 XP to level up from level 22 to 23. That is reflected in the blue XP bar in your profile. It fills with the XP accumulated and a full bar means 100%.