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 Material Design
<div id="dialog">Lorem ipsum dolor sit amet, consectetur adipiscing elit...</div>
<script>
$("#dialog").dialog({
modal: true
});
</script>
True Bootstrap 4
<div id="dialog">Lorem ipsum dolor sit amet, consectetur adipiscing elit...</div>
<script>
$("#dialog").dialog({
modal: true,
uiLibrary: 'bootstrap4'
});
</script>
True Bootstrap 5
<div id="dialog">Lorem ipsum dolor sit amet, consectetur adipiscing elit...</div>
<script>
$("#dialog").dialog({
modal: true,
uiLibrary: 'bootstrap5'
});
</script>
False
<div id="dialog">Lorem ipsum dolor sit amet, consectetur adipiscing elit...</div>
<script>
$("#dialog").dialog({
modal: false
});
</script>