Microsoft today announced Windows Timeline features on 2nd day of Build 2017. This feature allows one to go back in time and pick what one was doing before on other devices or on the same device.
It works similar to hand-off feature on other platforms. As per Microsoft the data of activity gets stored into cloud and with help of Cortana one can pick up from where he /she left off.
Windows Timeline works across platforms, as Cortana is available across iOS, Cortana, Android.
In case an app is not available on one of the devices and you want to pick up from where you left, Cortana will prompt you to install app on that device.