VFS
The abstraction layer that makes open() and read() behave identically regardless of which filesystem lies beneath.
The Virtual File System defines a common interface every filesystem implements. A program reading a file does not know, and does not need to know, whether it sits on ext4, on an NFS server in another country, or in a virtual filesystem such as procfs where the contents are created at that very moment.
The model comes from Sun Microsystems in the early 1980s and was built precisely to mount NFS seamlessly. The core concepts are the inode for file metadata, the dentry for the mapping between name and inode, and the superblock for the mounted filesystem. FUSE exploits the same layer to let filesystems be implemented as ordinary user-space programs, which is what made sshfs and its kin possible.