On most of the sub-sites, each post has its own dedicated page with a list of all the comment?s that have been added to it; this is known as a thread.
What is MetaFilter?
From the [OED]: "In Greek, the Meta prefix is joined chiefly to verbs and verbal derivatives; the principal notions which it expresses are: sharing, action in common; pursuit or quest; and, especially, change (of place, order, condition, or nature)".
And, from [Merriam Webster]: A filter is a "porous material through which a liquid or gas is passed in order to separate the fluid from suspended particulate matter".
So MetaFilter can be thought of as a shared pursuit or quest to separate the particulate matter of the web from the ubiquitous fluid it's suspended in. Put simply, it's a collaborative effort to point out (and discuss) the interesting stuff on the web.
MetaFilter runs on a ColdFusion engine written by MattHaughey. The source isn't available although he is thinking of GPLing it in July 2003. If you'd like to run your own MetaFilter-esque website, there are [plenty of open source engines] that fulfil many of the features (plus more, in some cases) that MetaFilter offers.
You can find details regarding MetaFilter's RSS feeds at RSSFeeds.
You can [find MetaFilter statistics here]. Also see MoreResources and RelatedWebsites.