The Universal Clipboard is exactly what it sounds like: Copy and paste that works across your iPhone and your Mac. You just use the copy command as you normally would, then pick up again on your other device with the paste command, though Apple warns users that “copied content is available to paste on your other devices only for a short time.”
So, say you’ve got an address up on your MacBook, in an email. Highlight the text, choose Edit and Copy (or hit Cmd+C), and then switch over to your iPhone, in an app like Notes. Tap and hold on the screen to bring up the pop-up menu, choose Paste, and your text should appear.
It works in the other direction too, of course, and with images as well—so you could, for example, paste something that you’ve got in Photos on your iPhone right into a document in Pages, no middle steps required.
From gizmodo’s handy tips for working seamlessly between your Mac and iPhone