Filesystem in Userspace (FUSE) is a software interface for Unix-like computer operating systems that lets non-privileged users create their own file systems without editing kernel code. This is achieved by running file system code in user space while the FUSE module provides only a "bridge" to the actual kernel interfaces.
FUSE is available for Linux, FreeBSD, OpenBSD, NetBSD (as puffs), OpenSolaris, Minix 3, Android and macOS. wikipedia 
FUSE is needed for certain services and applications that rely on the file system, for example Gocryptfs uses FUSE.