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An *.html.erb file is just an HTML file with some embedded ruby. Whenever you see a <%= some_code %>, that is ruby code to be executed and will show up on the page. Similarly, <% some_code %> is also ruby code, but will not show anything, just execute.
Here is a snippet from our the Indiana University production environment's app/views/modules/_header.html.erb, which you can see here: media.dlib.indiana.edu
The snippet sets up the featured items on the front page.
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