What is Redstone? It's the codename for the next big set of updates to Windows 10. Redstone 1 was released during the summer of 2016, while Redstone 2 is slated for early 2017. Rumor has it that Redstone 3 will be available in late 2017.
Windows 10 received an "Anniversary Update" during the summer of 2016 (codename Redstone 1) that introduced a number of new features and enhancements over the November Update released in late 2015. Up until now, this update was codenamed Redstone 1 and is the first of two major updates coming under the Redstone wave.
Redstone 2 is slated to bring new features and enhancements that did not make it into Redstone 1 in time. Currently, Windows Insiders are testing Windows 10 builds in the Redstone 2 branch, with an official release slated for early 2017.