What is w.fn.init[0]?

From time to time (normally when my code is wrong), I get to see this object reference in the developer console:

w.fn.init [form#addOnListFilters.form-horizontal]

The way I'm getting this message in the console is by submitting a form, and I have the following Js code:

   $('form[method="get"]').on('submit', function () {
                console.log($(this));
            });  

I'd like to know where is this w.fn.init thingy coming from to learn about it, but I don't know where to search it in the jQuery doc.