What does do in rails erb?

<%== is equivalent to raw.

From the Ruby on Rails Guide:

To insert something verbatim use the raw helper rather than calling
html_safe:

<%= raw @cms.current_template %> <%# inserts @cms.current_template as is %>

or, equivalently, use <%==:

<%== @cms.current_template %> <%# inserts @cms.current_template as is %>