I think it means the same thing as shadow banning. We have largely phased that out, except for spammers and serial trolls. For the most part we tell people that we banned them by posting a reply to one of their comments, and when they seem rehabilitable (I suppose that's not a word?) we invite them to email hn@ycombinator.com if they want to be unbanned.
However, vouching allows the community to rescue legit dead comments, and we review all the vouches. That means that if an account has negative karma (perhaps because of a faux pas early in its history), but has been posting good comments since then, we'll eventually see it. When we do, we use an internal feature code-named 'redemption' to put its karma back to 1 and restore any remaining dead comments.
Check your profile settings. There should be one called "Showdead." Turn it to "yes."
If it is already at "yes," then you may have just not noticed because it looks similar to when someone's comment has been flagged to death or downvoted into the negatives.
Apparently people will be allowed to say whatever they want but nobody will see it? I wasn't aware HN had this.