What does `:location => …` and `head :ok` mean in the ‘respond_to’ format statement?

render ... :location => @user will set the HTTP location header to inform the client of the location of the newly created resource (that is, its URL)

head :ok sets render to return an empty response (so just the header, no body) with status 200. head :ok is shorthand for render nothing: true, status: :ok.
Here’s a list of all the :status options you can use for setting the appropriate status code.