A perfect fourth is a music theory term. The guitar's strings (with one exception) are tuned in fourths (5 half steps). For example, going from E to A is an interval of a perfect fourth.
From E to A (going up on the two lowest strings) is a fourth, and all strings up to and including the G string are all at intervals of a perfect fourth. From the G string to the B string is an interval of a major 3rd, then it's another perfect 4th interval to the E string from the B.
Tuning 1 is a baritone tuning - in other words, it's standard tuning down a perfect fourth (or, 5 half steps). You'll hear this with Steve Vai's seven string guitar work, minus the high string. Baritone guitars have this tuning.
Tuning 2 is simply a standard tuning, down a whole step.
