What Tyberius wrote is correct.
Typically redacted information includes, but is not limited to, email and phone numbers.
Since redaction is one of those few processes that generally go by without anyone noticing, I'd like to add a few things though. The feature isn't that old (a couple of years maybe), before then, moderators had to raise tickets to remove such information.
There is a recommended practice (somewhere, I think and iirc) to first create a new edit entry and then remove the sensitive information from the past revision.
In most cases I dealt with I opted to radically edit the source post. Then I replaced the sensitive information with something like (redacted). I have not done it often, and it'll be a pain to find.
Doing it this way has the advantage that the sensitive information is not visible immediately anymore. Since it needs confirmation by a second moderator, it could be out in the open for a while.
Most of the times, however, the posts needing redaction are boarding towards spam.
There is, as stated before, not much information available anymore, but this could actually be our very first attempt at redaction.