What does a Universal Clipboard notification mean on Apple devices?

The short answer

A Universal Clipboard notification means something you copied on one Apple device is now available to paste on another nearby Apple device.

How does it work?

Universal Clipboard is part of Apple’s Continuity features. When you copy text, an image, or a file on one device, it automatically becomes available on your other Apple devices for a short time.

For example:

Copy a link on your iPhone and paste it on your Mac
Copy an image on your iPad and paste it into a document on your MacBook
Copy a password on your Mac and paste it on your iPhone

The notification appears to let you know the clipboard content has been transferred between devices.

What are the requirements?

For Universal Clipboard to work, all devices need:

Same Apple ID — both devices must be signed into the same account
Bluetooth turned on — devices communicate over Bluetooth
Wi-Fi turned on — both devices need Wi-Fi enabled (not necessarily connected to the same network)
Handoff enabled — go to Settings > General > AirPlay & Handoff
Supported devices — a Mac running macOS Sierra or later, and an iPhone or iPad running iOS 10 or later

When should you worry?

In most cases, this notification is completely normal and harmless. But keep an eye out if:

You didn’t copy anything — if the notification appears and you haven’t copied something recently, someone else may be signed into your Apple ID on another device
You’re on a shared device — be careful about copying sensitive info like passwords or credit card numbers, since it could appear on a nearby device
It’s happening constantly — an app may be writing to your clipboard in the background

How to turn it off

If you don’t want clipboard content syncing between devices:

Go to Settings > General > AirPlay & Handoff
Toggle Handoff off

This disables Universal Clipboard along with other Handoff features. There is no way to disable just the clipboard sync on its own.