Hero XP
Hero XP is the only way to level up your hero and would give buffs to your facility and to your hero and the higher the level of your hero is, the more XP it needs and the stronger it will get
Hero Energy
Hero energy can regenerate over night and the next day it would be full so you can sent it to fight Alien Invaders.
You can also use stim packs which can give 100, 500 and 1000 and this give your hero instant energy so if there is a invader challenge you can just use stim packs and attack invaders so you will get a better ranking and better rewards
Invaders
Alien Invaders are scattered across the map and range from level 1 - level 20 and the higher the level, the more Hero XP it gives but will drain more energy and the higher the level of the bug is it will have more health and if you kill a Alien Invader you will get rewards like armory chest, Stim Packs and more