This feature is part of Pick Up Where You Left Off which began appearing on Windows 10 Creators Update earlier this year but Microsoft is planning to take another step with Timeline in Windows 10 Fall Creators Update. When Timeline comes out, you will be able to continue working on websites and applications from one Windows 10 PC to another Windows 10 PC. For example, when writing an email on a laptop, you can continue writing that email when you switch to a PC.
Currently, when Timeline is temporarily postponed, some features will probably force you to wait longer. However, Windows Insiders will test Timeline immediately after Windows 10 Fall Creators Update is released to the public.
Part of the Timeline is already visible in the internal build of Windows 10 but currently the user experience is still not good. The design of effects and some other problems are far from what we see in Build 2017. The demo in Build 2017 is mostly just the ideal product and in fact will be quite different.
You can try the Windows Timeline experience now by downloading Cortana's latest beta update on Android here https://play.google.com/store/apps/details?id=com.microsoft.cortana . Don't forget to join the Cortana beta testing program ( https://play.google.com/apps/testing/com.microsoft.cortana ) to get this new feature.