Two separate Cumulative Updates are rolling out today. One for Windows 10 1511 and another for those who are still running the Windows 10 July 2015 version.
The one for Windows 10 1511 is delivering under KB3135173, while the other is under KB3135174.
I noted earlier today that Microsoft has used Patch Tuesday to christen its first-ever attempt at a change list for Windows 10. So, as the CU rolls out today, take some time to read through the list of changes, quality improvements, and security fixes included.
According to the history page, this update fixes a security problem that surfaced within the last month where InPrivate browsing for Microsoft Edge was actually caching visited URLs.