Partials in Rails can accept blocks. A block is a custom piece of markup. As with
regular Ruby blocks, you nest the markup within do...end section of the
render function.
<%= render layout: 'nav' do %>
<li><%= link_to 'Books', books_path %></li>
<li><%= link_to 'CDs', cds_path %></li>
<li><%= link_to 'Games', games_path %></li>
<% end %>Nesting works only if render specifies the partial with the layout: option instead of
the usual partial:.
The partial definition uses yield to mark the location where the markup from the passed
block will be inserted.
<ul class="navigation">
<%= yield %>
</ul>