How Messages for iCloud worksMessages in iCloud is here for iPhone, iPad, and Mac. This is how you turn it on.
With the release of iOS 11.4, Apple has at last unveiled Messages in iCloud: The feature encrypts your iMessage and SMS messages, keeps them in sync between your Apple devices, and backs them up. Messages in iCloud even lets you store your messages, photos, and other attachments in iCloud, giving you more local storage space.
How is this different than iMessage currently works?
How to turn on Messages in iCloud on iPhone and iPad
How is this different than iMessage currently works?
Currently, iMessages (and SMS messages, if you have SMS relay set up on your iPhone) get sent simultaneously to every device you own which you've also logged in to the iMessage system. But the system isn't perfect: Messages don't always make it to every device, especially if your device has been offline for awhile. On top of that, if you log into Messages with a new device without restoring from a backup, you'll only see n...
Source:
How Messages for iCloud works