This means that several programs can use the same DLL without having it loaded several time in the RAM (this would causes a very high memory usage if you have the shell32.dll loaded 50 times).
MS explained it here in this video:
Defrag Tools: #7 - VMMap
http://channel9.msdn.com/Shows/Defrag-Tools/Defrag-Tools-7-VMMap#time=03m35s