MetaFilter Projects is a place for members of the site to announce their latest web projects with the rest of the membership. Members can vote and comment on your project and may repost it to MetaFilter proper if a member deems it interesting enough to share on the main site. This subsite launched in November of 2005.
We're limiting the ability to post a project of yours to once every 30 days, and the thinking behind that is that the projects you choose to announce should be significant works—the types of things that typically take a few months to design, create, and build. Essays or blog posts aren't really what it's about here. We're talking larger stuff, like whole sites.
The rationale behind this is the projects page is merely a springboard for posts to the main MetaFilter site, if a member decides it's a project worth sharing there. The votes are an indicator of how much the membership likes your project, in the off chance it doesn't become a post, you'll at least know if it struck a chord with anyone, and you might catch a few comments for things people liked or that didn't make sense.
If you'd like to know more about the MetaFilter Universe, read the about page for MetaFilter.