declaring a variable without returning a value .

hey guys i am new to Jquery and JS in general and have a very simple question basically i came across a sorting script online and i tried to understand whats going on by going through the jquery documentation :

the script is as follows :

$(function () {
        var buttons = $('#fruit,#vegetable,#meat').click(function() {
            $(this).toggleClass('active');
            var classes = buttons.filter('.active').map(function() {
                return this.id;
            })
            .get().join(',.');
            $('div.fruit,div.vegetable,div.meat').hide().
                filter('.' + (classes || 'none')).show();               
                console.log(classes.get().join(',.'));
        });
    });

and this script is run on the following HTML :

<div style="float:right; padding:25px;">
            <button id="fruit" class="active"><span>fruit</span></button>
            <button id="vegetable" class="active">vegetable</button>
            <button id="meat" class="active">meat</button>
        </div>

        <div>
            <p>Trying to use buttons as an "or" case rather than "and." When choosing fuit or vegetable, I want to see tomato as part of each list, <em>not just</em> when both are selected.</p>  

        <div class="fruit">
            <p>apple</p>
        </div>

        <div class="vegetable">
            <p>pumpkin</p>
        </div>

        <div class="vegetable">
           <p>okra</p>
        </div>

        <div class="fruit">
            <p>orange</p>
        </div>

        <div class="meat">
            <p>beef</p>
        </div>    


        <div class="fruit vegetable">
            <p>tomato</p>
        </div>    

        </div>

fiddle here.

now i understand most part of the script except the following :

on line four of the script the author is using the following code :

                var classes = buttons.filter('.active').map(function() {

now button has not been returned any value , so should't it be empty ??

or is it that by saying the following (on line one):

            var buttons = $('#fruit,#vegetable,#meat').click(function() {

the author has also defined the button implicitly , as in :

var buttons = $('#fruit,#vegetable,#meat')

can somebody clarify this for me , as i am struggling to grasp this simple concept .

Thank you.

Alex-Z.