Threshold 2 is the internal codename for the Windows 10 Fall Update, which (as of 12-11-2015) has been released to the public. Threshold 2 is a relatively minor update to the July 2015 RTM build. It contains memory management improvements (included in Insider preview builds starting from build 10525), improvements to the overall UX and numerous enhancements to Microsoft Edge. The Insider Preview testing phase has concluded, and compilation of RTM candidates for the Fall Update has begun. The latest known candidate for release to the Current Branch (build 10586) was released to Insiders on the 5-11-2015, and made available for consumers on 12-11-2015. Its successor, codenamed Redstone, is currently in development.
Builds
Insider Preview
10540.0.th2_release.150903-1605 10546.0.th2_release.150911-2334 
10547.0.th2_release.150913-1511 10551.0.th2_release.150917-1700. 10568.0.th2_release.151010-1734 10576.0.th2_release.151019-1700 Fall Update RTM
10586.0.th2.151029-1602 10586.104.th2_release.160128-1934