This is a method generated for you by rails as a result of the
associtions declared in the Post and Comment models - in this case
belongs_to and has_many. Comments belongs to a post, and a post can
have many comments.
collection.build is one of these methods, and the API docs explain it
pretty well:
"collection.build(attributes = {}, …) Returns one or more new
objects of the collection type that have been instantiated with
attributes and linked to this object through a foreign key, but have
not yet been saved."
"collection" in this case is @post.comments
(None of of which has anything to do with namespaced resources...)