Unison is a tool to synchronize files and directories between different systems or within the same system. It can also be done between different platforms, for instance between a Linux computer and another computer with a different operating system, since it is a multi-platform program which is available for the best known operating systems.
An important point to understand is that it is not a backup tool. For instance if a file is deleted in one of the two replicas, it will be also deleted in the other one, since the diea is to maintain the two copies synchronized. If there are conflicts, for instance if a file has been modified in different ways in each of the replicas, unison will allow to reconcile the conflict, choosing one of the two versions over the other one.
It may have different applications, In my case, I use it to keep the computer that I have at home (and on the move, since it is a laptop) synchronized with the computer that I have in my office, which is a desktop computer. In this way, when I have to work at home I have to simply synchronize the laptop, and when I finish working at home I have to synchronize again so that my office computer is updated with the work I did at home. In principle, more than one computer could be kept synchronized, choosing in this case a reference computer and synchronizing the other computers against this one. I have used this system to keep a tablet and the laptop synchronized with the office computer.