https://arstechnica.com/gadgets/2018/09/windows-10-will-use-the-cloud-to-free-up-disk-space/

The next update to Windows 10, due to be released in October, will be smarter about how it frees up disk space and cleans up temporary files.

Central to this update is OneDrive cloud storage. Since last fall's update, Windows 10 has used a clever system of placeholder files to make OneDrive cloud storage seamless to use. With placeholders, the local OneDrive directory appears to contain the full directory structure and all the files that are found in the cloud. Any attempts to open or modify one of those files causes the data to be seamlessly retrieved from the cloud. There are various manual controls available; for example, files can be marked as always available offline (so they'll never be a placeholder, always a real file), and their local copies can be manually removed (leaving just the placeholder stub behind).