From a lowly roach that hides from the light, to the Bot in Shooter Game that knows when to shoot, and when to seek health or ammo; Behavior Trees are a powerful tool for creating Artificial Intelligence in Unreal Engine 4.
They're a combination of two asset types: The Blackboard and the Behavior Tree.
The Blackboard is the AI's memory. It stores key values for the Behavior Tree to use.
The Behavior Tree is the AI's processor. It makes decisions, and then acts on those decisions.
Below are links to the sections in the documentation covering the use of Behavior Trees. The Quick Start Guide takes you through making an AI chase the Player. Then there's the User Guide, which contains instructions on the most common tasks when using Behavior Trees. Next, the Node Reference contains a listing of all then nodes available in the Behavior Tree editor, what they do, and what their properties are. Finally, How Unreal Engine 4 Behavior Trees Differ is a paper written for those who are already familiar with how other Behavior Trees work.