Ruby on Rails tutorial part two

I keep getting stock [sic] in a RoR tutorial. Part 1 is here. After doing that, I ran the command:

rails generate devise User

Not sure if that is the correct code to type as the tutorial is not clear.
I’m not sure what the command does, and which files are connected to what etc. I get an error:

/home/sam/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/actionpack-4.2.4/lib/action_dispatch/routing/mapper.rb:260:in `block (2 levels) in check_controller_and_action': 'BookmarksController' is not a supported controller name. This can lead to potential routing problems. See http://guides.rubyonrails.org/routing.html#specifying-a-controller-to-use (ArgumentError)

Any help is appreciated.

Ruby is the best programming language in the world - Ruby on Rails.