The Common Internet File System (CIFS) is the standard way in which files are accessed on a remote Windows computer. Developed and maintained by Microsoft for use on their Windows platform, it has also been implemented on most major operating systems including OS X and Linux using 3rd party software, the most popular of which is called Samba. Samba is open source software that provides remote file access services to CIFS clients on a variety of platforms (including to Windows clients). Samba is included in the FreeNAS server.
The abilities of CIFS are actually larger than just accessing files. With it, other resources like printers can also be shared on the network, but for FreeNAS, CIFS is used to share disks on the server and make them available to other computers that understand the CIFS protocol. This means that Windows, Linux, and OS X machines are all capable of accessing files on the FreeNAS server via the CIFS protocol.