Is your company a union company?
If so, check if you have a right to bring a union representative the same way that your boss has someone from HR. Most likely, you do.
Because the one thing you want to avoid is being in a 2 vs. 1 situation that could turn into a "he said / she said" conflict.
Announce this before, and treat it as a perfectly normal thing. If pressed, explain calmly that since you don't know what the meeting is about, you are sure that the presence of the union rep is entirely unnecessary, but you brought him just in case, so no time is lost in fetching him if his presence should turn out to be of advantage.
That doesn't mean this is bad. It really could be entirely harmless, and the HR person might be there not to check on you, but to check on your boss. But the point is that you don't know. So better safe than sorry.
If you don't have a union rep on hand, then @Hilmar wrote a very good answer on how to prepare and behave, especially points #2 and #3.