Macworld has the details of what this means, but in short, as Macworld also points out in an earlier article on the topic, soon a stack of apps now available to download via the Mac OS X Downloads site will be available no longer - or at least, not through the new Mac App Store.
This is because of the various 'restrictions' Apple is placing on the Mac App Store, many of which echo the kinds of restrictions developers have become used to for the iOS App Store, including banning the release of software using private APIs, banning trial or demo versions of software and even banning software that changes the Mac OS X user interface, among many other restrictions as Macworld describes in detail.
Naturally, these apps will still be available elsewhere, and will still be installable, but for developers of now 'infringing' software that were used to be listed at Apple Mac OS X downloads page, they'll soon be enjoying that privilege no longer.
Macworld's latest article on the topic has a reprint of Apple's email to developers which is listed below:
Thank you for making the Mac OS X Download site a great destination with apps that offer users new ways to work, play, learn, and create on their Mac.
We recently announced that on January 6, 2011, the Mac App Store will open to users around the world, presenting you with an exciting, new opportunity to reach millions of customers. Since the introduction of the App Store in 2008, we've been thrilled with the incredible support from developers and the enthusiastic response from users. Now we're bringing the revolutionary experience of the App Store to Mac OS X.
Because we believe the Mac App Store will be the best destination for users to discover, purchase, and download your apps, we will no longer offer apps on the Mac OS X Downloads site. Instead, beginning January 6, we will be directing users to explore the range of apps available on the Mac App Store.
We appreciate your support of the Mac platform and hope you'll take advantage of this new opportunity to showcase your apps to even more users. To learn how you can offer your apps on the Mac App Store, visit the Apple Developer website at https://developer.apple.com/programs/mac/.
Vice President, Worldwide Developer Relations
So'¦ get your Mac OS X downloads from Apple's easy to use site while you can, for soon you'll have to go elsewhere if you want software that Apple doesn't want to promote, and presumably, doesn't really want to you to install.
Best of luck!