he's talking about your system's configuration and the layout of your directory structure you're trying to use to build FG... if you don't know the directory structure and where the necessary items needed to build FG are, then you're going to have a very long battle getting it built... in the download_and_compile.sh script, everything is housed under one directory which you choose... then other directories are created to store the repositories of the FG code in... additional directories are created for the cmake stuff and where those code files are written to so they can be used when building FG... finally there's the last set of directories where the various parts are installed so they can actually be executed and used...
FG == flightgear
SG == simgear - a part of flightgear that handles the simulation aspects
OSG == openscenegraph - this handles all of the drawing and graphics
plib == a small graphical interface library for menus and dialog boxes
the FG and SG parts are definitely needed to be built... they are version matched because differences between them can lead to a non-working sim... the other two parts are lower level code... over here, we build all four of the above but we could get by with not building OSG... we do so as to try to have the latest release so we don't miss any features in FG... we do already miss some, though, because of having older libraries that we cannot update on these systems...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."