PvE stands for "Player vs. Environment", it essentially means that the main focus of the map is to battle against mobs. This usually entails fighting your way through a dungeon, or trying to survive waves of enemies.
The Defenders is an arcade/PvE style minigame Minecraft map where you must defend a villager from zombies while on fortified battlegrounds. To assist you are lava pits, walls, cacti, fire, and cannons at your aid, which will help you protect the villager inside a castle.
Build and defend your own village. Kill mobs and get emeralds to trade/buy better and better equipment. Try to survive as many days as you can. Are you good enough to complete all 43 quests and get diamond rank? Have fun and by nice.