Apple released macOS High Sierra 10.13.5 on Friday, three days after rolling out iOS 11.4, tvOS 11.4, and HomePod 11.4. The update adds Messages in iCloud support, which was introduced earlier this week for the iPhone and iPad.
Messages in iCloud acts as a cross-device sync tool for your conversations. It stores conversations along with the photos, GIFs, videos, and other attachments they contain in your iCloud account online to save space on your iPhone, iPad, and Mac. Any conversation you delete from one device automatically delete from your other devices.
Until today, Messages in iCloud was limited to iPhones and iPads. With this macOS update, the feature is available in Messages on the Mac, too.
macOS High Sierra is a free update. You can install it by going to Apple menu > App Store and select the Updates tab.