You have a Mac, and maybe another Apple device like an iPhone, iPad or Apple Watch. If you have started a certain task on a device and want to continue on another, you can do it with Handoff.
This feature allows seamless transitions between any of your devices. Launch an email, text message, or other item on your Mac, then continue on your iPhone. Start a task on your iPhone and continue it on your iPad. You can even start a task on an Apple Watch and continue it on your iOS device.
Handoff works primarily with Apple apps such as Safari, Calendar, Contacts and Podcasts, but it also supports some third-party applications. On the front of the operating system, Handoff OS X requires Yosemite or later Mac, iOS 8 or later on iPhone or iPad, and watchOS 1.0 or later on Apple Watch. At the hardware end, the feature requires some models of iOS devices and Macs, but supports all Apple Watch.
In order to use Handoff, each device must be connected to the same Wi-Fi network and be logged in to iCloud with the same Apple ID. How to set up and use Handoff.