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
指挥官的经验值。会在每波结束时获得，也可通过摧毁某些建筑、神器等各种方式获得。升级时，你可以在波次结束时从一组奖励中进行选择。