English (Source)
Your Commander's experience. Gained at the end of each wave and through other various means such as destroying certain buildings, artifacts, etc. When leveling up you get to choose from a pool of rewards at the end of the wave.
Chinese Translation
你的指挥官的经验。通过每波结束和其他各种手段例如摧毁特定建筑、文物等获得。当升级时，你可以从奖励池中选择奖励。