I think it means just that. an ability to erect a "shield" around your mind. nothing comes in, nothing goes out - depending, of course, on the perfection of it. from what I've read in Trek novels, it's a routine exercise Vulcan children are taught. what is interesting is if Humans can be taught this as well. I've always assumed that it takes a certain kind of mental discipline and acumen to do that, as with the mindmeld. I think mindmelding is ALL about shielding or the lack of it.