That's the begging of a comment.
<!-- Comment goes here -->
Note: The comment tag is used to insert a comment in the HTML source code. A comment will be ignored by the browser. You can use comments to explain your code, which can help you when you edit the source code at a later date.
As for JavaScript if your comment is only one line long you would do this:
//Comment goes here
If your Comment were to go further than one line, you would use this method of commenting:
/* Comment goes here */
Also Javascript comments can only go between the "script" tags, however, nirchuck gave you an example of using the "script" tag. When using JavaScript, the proper way is like this:
<script type="text/javascript">
</script>
Use the type attribute otherwise not everything will work.
Hope it helps, Death