Controllers can be created from the Tool Bar on the left side of the interface.
Controllers are invisible layers that do not render in your exports. They can be used in a variety of use cases, but they are most commonly used to influence other layers with linking.
Just like any other parent layer, controllers can manipulate transformation properties (position, scale, rotation, shear) of their children layers. This makes them great tools to manipulate many layers at the same time without affecting keyframes set on each layer.
Additionally, controllers can have their own keyframes, allowing you to animate all the controller's children identically.
Other software sometimes refer to these layers as "Null Layers" or "Null Objects."