In the case of a flight simulator, it gives the documentation for the components which make up the sim: aircraft, scenery, effects, weather, ATC etc. It should also include the tools to build these where modelling/graphics/sound software don't actually have the facility already included. So for FSX there are compilers for scenery and aircraft, plugins for modelling programs, a tool to convert bitmaps to DDS format and suchlike.
Expect a lot of reading when the FSW sdk is released. A lot.