There is nothing more important in defense on the football field than protecting the red zone. It is where the vast majority of touchdowns are scored. Largely, defenses are judged by how well they keep opponents from getting into the end zone once they crack the 20-yard line—unless they are exceptionally bad at giving up explosive-play scores. Red-zone defense directly correlates with overall scoring defense, so if you’re good in the former area, it’s very likely that you’re good in the latter, as well.
Last season, the Pittsburgh Steelers had among the best red-zone defenses, one of only five teams in the league to prevent teams from scoring a touchdown on at least half of their trips inside the 20-yard line.