TrueNAS, formerly FreeNAS, is a tiny FreeBSD-based operating system which provides Network Attached Storage (NAS) services (CIFS, FTP and NFS). ZFS is its prime component, a combined storage management and file system.
Note the provider of this CORE image does not support this build for enterprise deployment -- production or mission critical settings one supposes. It's provided as courtesy to those with particular hardware driver needs. Otherwise people are encouraged to migrate to SCALE edition instead.