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 mediante otros medios como destruir ciertos edificios, artefactos, etc. Al subir de nivel podrás elegir de un conjunto de recompensas al final de la oleada.