OverlayFS is a Union-Filesystem, the goal is to take 2 FS (normally one Read-Only and one Read/Write) and use them both on the same mountpoint/directory. the Read/Write is called the Upper FS and the Read-Only is the Lower FS. This overlap means that the Lower FS will be overlooked if any new version of a file exist in the Upper FS. ^[1]

It is now possible to assemble an OverlayFS stack using file descriptors rather than path names.^[2]