I'm getting used to using FFS and love its simplicity, but there unfortunately appears to be a bit of a basic hole in the documentation of the program. I've looked high and low, on both of the SourceForge FFS pages, and can't find a bit of very basic information!
There are five icons that denote the five different categories of files. Hopefully the image below will work:

Of those five, I'm pretty sure that:
1 is when a file exists in the directory on the left but does not exist in the directory on the right
2 is when some file (which one?!) is newer than the other
3 is ???
4 is when some file (which one?!) is newer than the other
5 is when a file exists in the directory on the right but does not exist in the directory on the left
Again, I've looked through the all the support documentation I can find, and cannot find exactly what these icons mean! It's probably because the developer thought it was so obvious that he didn't need to spell it out, but I guess I'm dense enough I need it spelled out. :-P