The Saints are ranked #11 in total defense. They're actually #11 in rushing defense, and only #24 in passing defense. So it sounds like they're better at defending the run than the pass.
The 5.1 YPC stat is strange, given all of that. As it turns out, opposing teams have rushed only 135 times against the Saints, the lowest in the league. So teams are able to rush reasonably well, but they're just not trying it very much. Sounds to me like teams are having to give up on the run in order to pass and play catch up.
They're also #4 in scoring defense.
This is a good defense, which is helped by their incredibly efficient offense. The key is to do everything we can to keep their offense in check so we don't get too far behind too quickly, forcing us to give up on the running game.