Unison is a cross-platform synchronisation software. It allows synchronisation or backup of files in both directions. i.e. it will look for the newest version of each file and mirror that to both locations; if it is unsure, it will ask for each individual file.
In order to use it as a network backup solution, you need to enable it on FreeNAS, and install it on your PC to be synchronised.
To enable it, you need to enable SSH first:
Go to: Services -> SSH
Select Enable near the top right.
Enable options Permit root login, Password authentication, TCP forwarding using default TCP port of 22.
Click Save and Restart
Then enable Unison:
Go to: Services -> Unison
Select Enable near the top right.
Select the Share you want to synchronise (i.e. the network backup location on FreeNAS server).
Click Save and Restart
Then you need to set up the Unison Client on your PC which has the folder to be synchronised.