What is the origin and purpose of the variable $data in KnockoutJS?

Question

In the KnockoutJS tutorials I stumbled upon the following code example that contains an unexplainable variable $data.

The View (html):

Answer 1

i made it work

.selected {
    color:red;
}




var viewModel = {
    // Data
    folders: ['Inbox', 'Archive', 'Sent', 'Spam'],
    selectedFolder: ko.observable('Inbox'),

    // Behaviours
    selectFolder: function (folder) {
        this.selectedFolder(folder);
    }    
};

window.mailViewModel = viewModel;
ko.applyBindings(viewModel);

please take a look at http://jsfiddle.net/bowen31337/48RDd/