This skinpart is used to show the most recent entries of your weblog. It's usually the home page of your site.
Very basic buildup for a main index:
<html>
<head>
<title>My Weblog</title>
</head>
<body>
<h1>My Weblog</h1>
<%blog(mytemplate,20)%>
</body>
</html>
This will show the 20 most recent items of the default weblog (unless overridden), using the 'mytemplate' template.