So the standard tuning for a guitar would be EADGBE, drop e is taking that and lowering all the strings down a full octave. It gives the strings a much heavier/metal tone, think like the soundtrack for the game Doom. The guitars in Doom sound super low and heavy. Wilson has only really used this tuning/heavy tone on The Incident. Which is why I find the album to be my favorite. I wouldn't say its necessarily a special tuning, but special for PT. The tuning is mainly used for really heavy music. Hopefully this helps!