Hi chicagorandy, like tiger1962 said, a sim “depends†(relies) on these files to run. An example is a Train sim that I have. When installing a particular train route, or “layout†if you will, there might be a certain file, say a boxcar or caboose file, that the sim needs for that to be properly represented. So, the file is called a dependency.
Just guessing, but in the case of MSFS a “dependency†could be a certain engine cowling, prop blade, tree, water texture, anything really...
Mike