To understand it,
custom_fonts/font is a property of the label which will get the dynamic font resource.
extra_spacing_char is a property of the dynamic font resource.
In order to access the dynamic font from the label it needs to be returned via a method which is what the brackets are so
get_children(), etc are all methods that belong to specific node types.
The properties are the values that can be accessed from that node or resource so
get("custom_fonts/font") returns the resource and now it has access to the
Also properties can have set and get like
Node2D.get_position() which is the same as