The #type property specifies that the array contains data that needs to be rendered using a particular render element. Render elements are plugins (yes, plugins) that encapsulate a defined renderable component. They essentially wrap another render array, which can use a theme hook or a more complex render array to process the data they are responsible for rendering. You can think of them as essentially standardized render arrays.
There are two types of render elements: generic and form input elements. Both have their respective plugin types, annotations and interfaces. They are similar in that they both render a standardized piece of HTML; however, form input elements have the complexity of having to deal with form processing, validation, data mapping, and so on. Remember, when we defined our form in Chapter 2, Creating Your First Module, we encountered arrays with # signs. These were (form) render elements with different options (properties).
To find examples of these two types of render elements, look for plugins that implement the ElementInterface and FormElementInterface interfaces.