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.
Spanish Translation
La experiencia de tu Comandante. Se gana al final de cada oleada y a través de otros medios diversos como destruir ciertos edificios, artefactos, etc. Al subir de nivel, puedes elegir entre un grupo de recompensas al final de la oleada.