Judges are like umpires because they "should not be rooting for one team or another"--we expect judges to have umpires' neutrality and to be fair to both parties. And, just as umpires are not the most important people on the baseball field, judges are "not the most important people in our democratic system of government." Judges have an important role to play in "policing constitutional boundaries" but that is a limited role--the policymakers are the people and their elected representatives."
But the metaphor doesn't work, Kagan says, because it "could suggest that law is a robotic enterprise," with the decisions "all clear cut and no judgment" needed. "That's not right," especially on the Supreme Court level.
"Judges do have to exercise judgment" and that is not easy. They must apply "the law and only the law"--not their personal or political views, but not every case is decided 9-0 because "reasonable people can reasonably disagree" about how to resolve them. In may cases there is a "clash" between different constitutional values; judges sometimes are not going to agree about the appropriate outcome.