What is a level?
Levels behave like folders in a file system, or a container for nodes.
By default levels are just empty shells, but they can be customised to receive inputs and outputs, and further modified by parameters set directly on the level nodes. They provide an interface to the outside, and a way to communicate between the nodes on the outside and inside.
Levels can be nested the same way folders can be nested too. As you can imagine, combined with the interfaces, they can be incredibly useful to organise the structure.
Levels are very important in maintaining a modular design of your node graph, instead of laying all nodes flat on the root level(/), self contained sub graphs can be put inside level nodes to keep the main graph clean and tidy.