Experience
Experience (XP) is one part of the in-game reward system of King of the Hill. The amount of XP earned is used to calculate the level of a player. The level then is used to unlock more and better weapons as well as vehicles (see equipment for mor details).
The progress of the players progress can be inspected in the bottom right corner of the players HUD, where a green bar is progressing to the right. It shows the currently earned XP since the last level up as well as how much XP is needed for the next level.
The total earned XP (over the whole lifetime of the game) can be seen in the player menu.