It goes in the html header on the index.php I think. Or the index.template.php for the theme(s).
When you check out the index.php for the forum, the first bunch of lines are this:
<html xmlns="http://www.w3.org/1999/xhtml"><head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
<meta name="description" content="" />
<meta name="keywords" content="PHP, MySQL, bulletin, board, free, open, source, smf, simple, machines, forum" />
<script language="JavaScript" type="text/javascript" src="https://www.acidmods.com/forum/Themes/default/script.js?rc2p"></script>
<script language="JavaScript" type="text/javascript"><!-- // --><![CDATA[
var smf_theme_url = "https://www.acidmods.com/forum/Themes/AcidMods_Red_Theme_RC1";
var smf_images_url = "https://www.acidmods.com/forum/Themes/AcidMods_Red_Theme_RC1/images";
var smf_scripturl = "https://www.acidmods.com/forum/index.php";
// ]]></script>
<title></title>
Cyber, all you need to do is edit it where it says
<title></title>
and put something in there for the title of the site. That will show on all pages.