9p is a simple network protocol for implementing filesystems. It originates from the plan9 operating system but its main usefulness nowadays is that it permits

9p is a simple network protocol for implementing filesystems. It originates from the plan9 operating system but its main usefulness nowadays is that it permits people from implementing filesystems in user space without having to integrate deeply into kernels specific interfaces like FUSE. I.e. if your OS can mount 9p volumes you can implement a user filesystem easily by just running some process on localhost and mount it. And it also works remotely (although that's less useful IMO, although not entirely useless)