Using void(0) is an alternative to setting href="#" which scrolls the page.
When using
<a href="javascript:void(0)">Link</a>
you get a link that does nothing. It doesn't scroll anywhere nor does it change the location bar in any way. This is useful to add an onclick handler that does the real work:
<a href="javascript:void(0)" onclick="someAction(this)">Link</a>