Metafilter is a weblog (what's a weblog? | comprehensive history of weblogs) that anyone can contribute a link or a comment to. A typical weblog is one person posting their thoughts on the unique things they find on the web. This website exists to break down the barriers between people, to extend a weblog beyond just one person, and to foster discussion among its members.
If you're new to the site, I'd suggest taking a look around, checking out the archives, and getting a feel for the place. You might also consider registering as a member. Members can post comments, customize the look and behavior of the site.
After becoming a member, check out some of the links and think about leaving a comment or two. If you stick around for a while, you'll get a feel for what types of things are posted as links, and if you find something amazing and/or enlightening, please post it.
The privilege of posting links to main page comes after posting a few comments and being a member for at least a week. This lag is built in to allow new members to get used to the place and to understand what other members consider good links. With the current number of members, even if fewer than 1% of the total membership post a link each day, it's far too many links to take in. Please take extra special care when selecting a link for the front page. To get an idea of what constitutes a good link, take a look at the link guidelines, to hear some philosophies guiding the site and community, check out the new user message.
To learn more about the history of MetaFilter and hear some Best Of stories, watch this video from May 2010's GEL Conference:
Terms used here
Thread: A thread (sometimes called a post) is one of the main messages
you see on the MetaFilter homepage. These are the starting points for
discussions, and are ideally unique, interesting, valuable links
accompanied by commentary that starts an engaging conversation.
Comments: These are the heart of discussion at MetaFilter. Once a thread
is started on the homepage, any member can contribute their voice to a
conversation by posting comments. The best comments often include not
just the member's opinion on a subject, but new links that relate to the
topic being discussed, or personal experiences that might let other
members see the topic in a new light.
MeFi: It's shorthand for MetaFilter.
The kind of site you can feel so comfy with, it gets a nickname.