Normally, Microsoft releases patches for its software products on the second Tuesday of each month, also known as Patch Tuesday. However the company decided to release a patch outside its normal schedule this week, specifically for Windows 8.1 as well as Windows RT 8.1 and Windows Server 2012 R2.
The patch is described on a Microsoft support page, which says simply that the file is supposed to improve "the reliability of Internet Explorer 11." The page says that the file is available from Windows Update, but there are also manual download patches available for the 32-bit and 64-bit versions of Windows 8.1, along with another patch for Windows Server 2012 R2. The patch for Windows RT 8.1 is only available via Windows Update. While Microsoft is keeping details of what this IE11 patch fixes to itself, the support page does say that users must restart their PC after it is installed.
So far, only the Windows 8.1 version of IE11 has gone beyond a public preview build. Microsoft has yet to launch a non-preview version of the web browser made for Windows 7, but it has issued a IE11 Automatic Update Blocker Toolkit for that OS, which is usually a sign that the final release is close at hand.