Without any arguments, yield will render the template of the current controller/action. So if you’re on the cars/show page, it will render views/cars/show.html.erb.
When you pass yield an argument, it lets you define content in your templates that you want to be rendered outside of that template. For example, if your cars/show page has a specific html snippet that you want to render in the footer, you could add the following to your show template and the car_general layout:
show.html.erb:
<% content_for :footer do %>
This content will show up in the footer section
<% end %>
layouts/car_general.html.erb
<%= yield :footer %>
The Rails Guide has a good section on using yield and content_for: http://guides.rubyonrails.org/layouts_and_rendering.html#understanding-yield
The API documentation for content_for is helpful too and has some other examples to follow. Note that it’s for Rails 3.1.1 , but this functionality has not changed much since 2.3, if at all and should still apply for 3.0.x and 3.1.x.