It means its only viewable in the network computers at msn's headquarters or office buildings.
Its not made public for everyone to see on the web.
--------------------EDIT--------------------
I found what I was looking for. After many hours of extensive Google searching and only a handful of results I managed to squeeze out the info I wanted. Since these sites are hidden there is very little info on them, thus making it very hard for me to discover. But! I have, and now that I have it seems easy to find them
The Tor Network -
http://tor.eff.org/ - ex. of web address:
http://6sxoyfb3h2nvok2d.onion
The I2P Network -
http://www.i2p.net/ - ex. of web address:
http://search.i2p/
If you already know about these then don't spoil my fun by saying "This is old news" bla bla bla, this is the first time I've seen this and I think its pretty kool.