How does form_for work for linked models?

Question

Ruby on Rails

Newbie on rails. Explain how form_for works in this case?

<%= form_for([@post, @post.comments.build]) do |c| %>

What's in the object @post? Why @post.comments.build?@postis it taken from the controller when the submit button is clicked?
There is a post with comments. The modules are linked, as usual:
Comment belong_to Post
Post has_many Comment