Branches lead to leaves,
Each leaf a single action,
Fluttering in wind

Behaviour trees are used to determine what the AI should do. They are very much like a tree in a literal sense. The decision starts at a 'root' and spans out until it finds a 'leaf' which will contain the action for the AI to take¹. The path which leads to the 'leaf' picked, is determined by composite and decorator nodes. There are also other ways to determine the behaviour of an AI, such as: Utility based AI and Finite State Machines.

1 Chris Simpson. Behavior trees for AI: How they work. 2014. [Online]. Available: http://www.gamasutra.com/blogs/ChrisSimpson/20140717/221339/Behavior_trees_for_AI_How_they_work.php [Accessed: 14- Feb- 2019]