Why does is say my photo stream will be shut down? What does this mean?

Click here ➜ Information about the My Photo Stream shutdown - Apple Support

You can use iCloud Photo Library to transfer images to a computer if needed. That is not going away.

Loss of My Photo Stream means we'll need to use AirDrop or cable transfer to get photos f rom our iPhone's Photos library to our Mac's Photos library. Only a small inconvenience.