Both Windows Timeline and Sets are upcoming Windows 10 features. The two features were previously announced by Terry Myerson and it was said that the new features would be coming to Insiders with future Insider builds.
In the last Windows Insider webcast, Microsoft provided more information on the time we can expect these new features. Jason Howard from the Microsoft Insider Team confirmed that Windows Timeline will be part of the next Windows 10 update (aka Redstone 4). Nevertheless, the Sets feature won’t be part of this update.
With Windows Timeline, Windows Insiders will get a really nice feature to try out soon (probably on Thursday).
Source: Windows Insider Podcast