$.text(["someText"]) - What does it mean?

jQuery.text does the heavy lifting for the implementation for the .text() method -- it seems to be an undocumented function with the core functionality for .text(), but missing some jQuery polish.

It's "imported" from Sizzle, where it appears as Sizzle.getText.

Inspecting the jQuery source will reveal that the $(selector).text() that you're familiar with, uses $.text internally:

jQuery.fn.extend({
    text: function( value ) {
        return jQuery.access( this, function( value ) {
            return value === undefined ?
                jQuery.text( this ) :
                this.empty().append( ( this[0] && this[0].ownerDocument || document ).createTextNode( value ) );
        }, null, value, arguments.length );
    },

It is an undocumented function (which means further jQuery revisions may drop it at will, without notifying you). You'll find its definition as such:

jQuery.text = Sizzle.getText;

Sizzle.getText, in turn, is documented as "Utility function for retrieving the text value of an array of DOM nodes". Seeing as Sizzle.getText is a documented feature, I would recommend using that rather than the jQuery shorthand, as I don't expect jQuery to drop Sizzle any time soon.

This function, then, is the piece of code that yields the text content of a DOM node. Your sorting method is sorting DOM nodes by the alphabetical order of their text content. I don't know why the author has decided to get the text of an array containing only one element ([a]), rather than passing the element immediately (a), which would work equally well.

After looking at your jsfiddle it appears it's a function for getting the text from an element, simular to .text()

console.log(a) logged <td>28/02/2013</td>

While

console.log($.text[a]) logged 28/02/2013