It is similar to Apple's Continuity function but for reasons unknown Microsoft is delaying it a second time. Timeline and PUWILO were originally supposed to be in the Creators Update this Spring, and now there's confirmation it won't make it into the Fall Update either. Microsoft on the other hand denies it represents a delay and claims it never suggested the features would definitely be part of the Fall Creators Update.
Responding to a comment by The Verge's Tom Warren on Twitter, Belfiore confirmed that the feature had been bumped but denied it represented a delay: 'Timeline won't be in the Fall Creators Update. We're planning for it to be in early insider builds shortly after FCU is out. We didn't "delay" it... It just didn't make the Fall release. At build we tried to roughly show the work in order of timely-ness with cloud files the first firm feature, then timeline and cloud clipboard next. Tried to convey timing wasn't specific on these. [...] From my POV we described [a] set of features that would come *starting* with FCU .. and I thought we'd communicated uncertainty. Our dilemma is how to show future work and describe our direction.. it's good to do that! [We] Need to get better at conveying [that] timing will vary.'