If set to true, the dialog will have modal behavior. Modal dialogs create an overlay below the dialog, but above other page elements and you can't interact with them.
Examples
True
<div id="dialog">Lorem ipsum dolor sit amet, consectetur adipiscing elit...</div>
<script>
$("#dialog").dialog({
modal: true
});
</script>
False
<div id="dialog">Lorem ipsum dolor sit amet, consectetur adipiscing elit...</div>
<script>
$("#dialog").dialog({
modal: false
});
</script>