What does a Ruby string wrapped in _() mean?

Question

I'm looking at a Sinatra app for a CAS server for SSO. I'm unsure what the underscore and parentheses mean in the string assignment. Here's a sample line:

@message = {:type => 'confirmation', :message => _("You have successfully logged in.")}

But it's also used in ERB:

<%= _("Username") %>

Answer 1

_ is the name of the translation method of GetText.

So _ basically means: Take the following string and use it as a key to find a localized/translated version.