Energy Armor is one of the traits in Loop Hero, it is basically the same as Magic HP which you will be familiar with if you have played as either the Necromancer or Rogue class and have the Arsenal Golden card active so you can access the Amulet equipment item which has Magic MP as its primary stat. Magic HP shields you from incoming damage and will fully replenish between battles, it is also unaffected by Landscape cards such as the Sand Dunes card which lowers all creature's HP by 1%.
The term Energy Armor is a bit misleading in Loop Hero as it is just a term referenced in one of the traits you can get as a Necromancer and not some kind of powered up suit of armor like the name suggests. When you play as the Necromancer every skeleton you summon will add 0.5 to your Energy Armour total. If your Attack Speed and max skeletons count is high enough you will be able to continually summon skeletons and quickly increase your Magic HP. This will allow you to easily vanquish the most powerful enemies in Loop Hero.
Check out our How to Customize your Deck
guide to get some tips on how to take advantage of the cards you get in Loop Hero.