It refers to very early 1980s CD manufactured by Toshiba, which had a large black triangle on the lettered face of the disc itself. Black Triangles aren't specific to Maiden. They are highly sought after by collectors, and expensive to buy now, but as my post to which Forostar linked points out, several early U.S. and European Maiden releases have exactly the same mastering. Some audiophiles believe that Toshiba's plant had the highest quality control at the time, and deduce from that they sound better. I think that's hooey. It's digital, not analog, so the difference in "pressing plant" is largely unimportant. As long as you don't have a defective burn from another manufacturer, assuming the mastering is the same, the music will sound exactly the same.
I would also add, for those who are interested, that many including myself believe that the original U.S. mastering of the debut Iron Maiden album actually sounds slightly better than the Japanese, and you also get Sanctuary as a bonus track (it was not included in the original Japanese or UK release).