The new sharing option appears in the bottom row of the iOS Share Sheet, and allows users with iCloud Photos enabled to copy an iCloud.com URL link to one or several selected images that lasts for 30 days.
Clicking a link in an email takes the recipient to a web page containing the image in question, as well as an option to download it. Sharing a link in the Messages app also shows a preview of the image right in the chat thread. The link also includes the author's name, the photo's title, and associated EXIF data, excluding location information.
The addition should come in handy for times when you quickly want to share an image from your iOS device, minus the hassle of uploading it to a third-party service. It's also more than likely that the iCloud link sharing option will feature in macOS Mojave.
For more on all the changes coming in iOS 12, check out our dedicated roundup.