"What speech, she said, comes close to being a 'true threat' but is so 'supervaluable' that we need to be worried about it?"

"She" = Justice Kagan, described in "Justices hear 'true threat' protected speech case" (SCOTUSblog). 

At the end of nearly two hours of debate, the justices generally appeared skeptical of Colorado’s contention that courts should use an objective test, that looks at whether a reasonable person would regard the statement as a threat of violence.... 

Chief Justice John Roberts... cited one of the statements for which Counterman was convicted, in which he told Whalen that “staying in cyberlife is going to kill you. Come out for coffee.”....

Justice Amy Coney Barrett... asked “[Who] is the reasonable person?” She outlined a hypothetical involving a college classroom in which a professor, for “purely educational” reasons, “puts up a picture of a burning cross and reads aloud some threats of lynching that were made at the time.” “Maybe it’s the case,” Barrett suggested, “that nowadays people would be more sensitive to that and … a reasonable Black college student sitting in that classroom would interpret that as threats … that might materialize into actual physical harm.”