Pro tuning is fairly simple to understand
It's only required if you actually use chord notation

Default tuning is E standard (E A D G B E), which is entered as: 0 0 0 0 0 0

For every 1, you alter the tuning of the particular string by a half step (semitone)
Positive numbers raise it to a higher tone, and negative number lower it

Here are some of the more common tunings:
Drop D (D A D G B E): -2 0 0 0 0 0
D Standard (D G C F A D): -2 -2 -2 -2 -2 -2
Drop C (C G C F A D): -4 -2 -2 -2 -2 -2
Open G (D G D G B D): -2 -2 0 0 0 -2

Many thanks to PikminGuts92 for this information