ZFS is a combination of actual file system like Ext4, LVM and MD (software raid) subsystems on Linux, with extra features on top which you unlock when these are a single system.

Due to licensing, it can't be included in Linux kernel directly, and so it's not seeing the same level of scrutiny like the rest of the kernel, but it is arguably used in production more successfully than btrfs (comparable feature set, in-kernel, but not maintained as well anymore).