I think he was refering to tuning the bass to Eb or Bb, for instance, because a lot of the songs are in black keys, so it'll be easier to play if you tuned you bass flat....
....On that note, they have gadgets that you can buy for your B or E string that'll allow you to get up to a whole step flat, when you flip the lever. I'm not an expert, but I don't see much use for those, and I'd rather not tune my bass flat, because the experts that I look at can do phenomenal stuff without tuning their bass flat! I say practice playing with it normal, eventually, you'll get the hang of playing songs with the standard setup, no matter what key they're in.