The bug caused Wi-Fi connectivity to drop intermittently on certain configurations of Macs and routers. Over the course of the month since Yosemite was released, the bug has generated a 1,000-plus reply thread on the Apple support forum.
Mac users that have experienced Wi-Fi performance issues running Yosemite should expect to see an improvement with this update. I've tested it on a system that was affected by the Wi-Fi bug and the problem now appears to be fixed.
According to Apple, the 10.10.1 patch not only addresses the Wi-Fi bug, but a raft of other issues too:
Improves reliability when connecting to a Microsoft Exchange server
Resolves an issue that may prevent Mail from sending messages through certain email service providers
Addresses an issue that prevents connecting to remote computers using Back to My Mac
Resolves an issue where sharing services, Notification Center widgets and Actions may not be available
Addresses an issue that could cause Notification Center settings to not be retained after a restart
Addresses an issue that might prevent the Mac App Store from displaying certain updates
Addresses an issue that could prevent some Mac mini computers from waking from sleep
Resolves an issue that might prevent Time Machine from displaying older backups
Addresses an issue that might prevent entering text in Japanese
There are also enterprise-related improvements:
Allows you to append search domains for partially qualified domain names when performing DNS lookups
Addresses an issue where the Mac App Store might offer an update to Apple Remote Desktop when the latest version is already installed
The update is available for Macs running OS X Yosemite through the Updates tab of the Mac App Store. More information on this update can be found here.