Winning a set without losing a single point. Not just 6-0, but winning every single one of the minimum 24 points needed. This is extraordinarily rare. It's happened only a handful of times in professional tennis history, including once at a Grand Slam: Yaroslava Shvedova took the first set against Sara Errani at Wimbledon 2012 without losing a point, still the only golden set in a Grand Slam main draw. A golden set means your opponent literally could not win one point against you for an entire set. It's tennis perfection.
"24 points played, 24 points won. A golden set. Has that ever happened before in this tournament? No. No it has not."