The jQuery .change() method uses the JavaScript change event to run a function. In otherwords, when the selected element changes value, the event handler kicks in and your functon runs. This is used a lot with input elements, such as dropdown options. When the user changes the value of the drowpdown option, your function starts and you can manipulate other elements.

An example would be that you page has different options for what kind of movies the user wants to look at. When he chooses Drama from the list, you have items related to that appear on the page.

For example:

$("#genre").change(function() {  // when the value for the element id="genre" changes run this funtion
    $(".choice").addClass("inactive");  // hide all elements of class="choice"
    var currGenre = "#" + $("#genre").val( ); // get the new value for the element id="genre"
    $(currGenre).removeClass("inactive");   // display the selected value

    <select id="genre">
        <option value="thrillers" selected="selected">Thrillers</option>
        <option value="romance">Romance</option>
        <option value="drama">Drama</option>
        <option value="comedy">Comedy</option>
<h2 id="thrillers" class="choice inactive">THRILLERS</h2>
<h2 id="romance" class="choice inactive">ROMANCE</h2>
<h2 id="drama" class="choice inactive">DRAMA</h2>
<h2 id="comedy" class="choice inactive">COMEDY</h2>

The Result: