It means that this is a good quality site to have links on. Anything above 4 is a pretty high quality site. Even at a 4 is a good place to be. However, just having a link in your profile does not make it a PR6 link. PageRank diffuses through a site from the top down. In other words, the main page may be a six, but your individual hub may end up a PR1. Those aren't specific numbers, more like a general example. But, that's pretty much what that information is good for.