Just now a Safari icon appeared to the left of my Dock with a tooltip saying it was from my iPhone. I have never seen this before! What is the point of it, why has it appeared today for the first time, and why has it now disappeared again?
This is Apple Handoff in action:
With Handoff, you can start something on one device (Mac, iOS, or Apple Watch) and then pick it up on another without losing focus on what you’re doing. For example, look at a webpage on your iPhone, then pick up where you left off in Safari on your Mac. You can use Handoff with many Apple apps—for example, Calendar, Contacts, Pages, or Safari. Some third-party apps may also work with Handoff.
(Image and text attribution: Apple.com)