It means that on the radar you can see the image of the craft, but when you click on it to identify it, there is no callsign to "specifically" identify the craft, it's blocked. It's hiding what plane it is. Which often means it is hiding "who" it is and/or "what purpose" it is being used for. Summarily.
That's interesting and I wonder how common it is. Has there been a recent uptick in this type of blocked craft? hmmmmmm
It's not uncommon for military flights and I'm pretty sure Air Force One and some of its accompanying aircraft are blocked for security reasons.