It's a new month, and that means a new security update for Android. The November Android security patch is out, and this is more noteworthy than most with its fix for the high-profile key reinstallation attack (KRACK). It also puts some Band-Aids on the newly released Pixel 2 phones.
Google actually released three "November" security patches last night. First there's the normal "2017-11-01" security patch, which contains all the monthly patches developed on the regular schedule. OEMs get these fixes a full month in advance (so early October in this case), and Google lets the bugs sit around for a whole month, allowing OEMs to port the patch to their phones. A month later, Google discloses the bugs and (theoretically) does a simultaneous release with the Android OEMs and carriers.
Anything higher than the "20xx-xx-01" release exists because Google needed to target a particularly nasty bug in the middle of the month and fast track it through the release system. In this case, we have the "2017-11-05" and "2017-11-06" releases. While the 11/5 patch was mostly about fixing a critical Qualcomm bug, it's the 11/6 patch that will get the most attention, since it patches the KRACK Wi-Fi vulnerability.