what does '$' mean in a .rb file?

I am embarrassed to have to ask what is probably a very simple
question, but, I've come across some code in a library that says this:

logger.debug("$language_id = #{$language_id}")

what does putting $ in front of a variable do?

http://ruby.activeventure.com/usersguide/rg/globalvars.html

Best regards

Peter De Berdt