macOS High Sierra brings new technologies that make it more reliable, responsive, and ready for future innovation. And refinements improve the features and apps you use every day.
Optimized Storage in macOS Sierra and later can save space by storing your content in the cloud and making it available on demand. When storage space is needed, files, photos, movies, email attachments, and other files that you seldom use are stored in the cloud automatically. Each file stays right where you last saved it, and downloads when you open it.