$msg.get

Hello all, What is $msg variable, and where can I read more about it? (is not in xwikivars.vm...) And how does $msg.get() works? Please, I really need some help. I want to include support for languages in my datepicker and Sergiu told me to put the strings in ApplicationRessources.properties and now I'm trying to get them in a js file with $msg.get("core.datepicker.somethingdefinedbymehere"), but it seems that the server does not know how $msg is, but it knows who $xwiki, $context and all other are.... So please, please, I would like to know more about this variable or how to get the strings from ApplicatonRessources. I'd really appreciate some help because I'm stuck on this.... Thank you very much, Evelina