They were talking horribly about her and had no idea she could hear them. Her response was perfect.
Wouldn't it be great if the whole "mean girls" thing ended in middle school? But it doesn't. Unfortunately there are moments that can shock and hurt us as adults just as much as when we were teenagers. The good news is that now we are old enough to take the high road and actually learn some surprising things. Right? Make sure you watch the entire video. It all comes full circle at the end.