PvE means player vs environment (non real player things in the game). This includes things like Monsters, Resource Nodes, Titans, Raids and other game generated targets. 

Let's break down what these things are!

Monsters 

There are 3 basic monster types in Kingdoms of Heckfire (Goblins, Skeletons, Treants) each monster type dwells on a specific land type also known as biome. 

Goblins = Grasslands
Skeletons = Badlands
Treants = Swamplands

Titans

These are the  larger and more powerful versions of the basic monsters. Again there is a different type based on the biome!

Resource Nodes

 These are nodes found around the Realm Map that players can harvest to gain Ore, Food and Gold. 

Raids and Raid Monsters

Raids are collaborative events that your clan can participate in.  To win the Raid your clan will need to defeat all the Raid monsters and fulfill all the objectives the Raid puts in front of you.

Leaders of a clan can start a Raid through the clan menu. Starting a Raid requires 1000 Clan Raid Summon Energy. These can be acquired by the clan doing certain quests that appear in the Quest Board.