At the launch of Dawntrail's Early Access, it was discovered that, to facilitate the new Blacklisting features, each service account's unique identifier is being broadcast to everyone within the zone you're in. This unique identifier is used to identify the player on a service account level, meaning every character on that service account shares the same unique identifier.
Some unhinged people created a tool that farms that data to create a database, allowing the users of said tool to track players on a per-user level. This also lets you know which retainer belongs to which player, which de-anonymizes the market board to a great extent and opens the door to harassment.
After the issue was ignored for a long while,
originally a statement was made here, basically asking them to kindly stop.
This, of course, did nothing, and there was the outcry that the issue was at a system level. Because it is. So in a Live Letter leading up to 7.2, they announced that they would be implementing measures... for 7.2. Which was still like a month away. People weren't happy with it, but hey, atleast they were doing something, right?
Fast forward to 7.2's release. They delete everyone's blacklist to accomodate for the new system. In reality, all they ended up doing was adding an obfuscation layer to the unique identifier, which was broken within 24 hours. The problem is that this is being handled client side at all, which many users commented on at the time as being the worst possible implementation possible. So the obfuscation layer is fully cracked, the tool updated, and we're basically right back to where we started.
As for what this means; it means there's a database that keeps track of who you are across all your characters and retainers, which can be used very easily for cyberstalking :)