This is used to get and set special data related to the monster.
With getmobdata, the array given will be filled with the current data. In setmobdata
the indexes in the array would be used to set that data on the monster.
Parameters (indexes) are:

0 = class (big, small, normal) 7 = y 14 = hair style 21 = weapon
1 = level 8 = speed 15 = hair color 22 = shield (again)
2 = HP 9 = mode (see doc/mob_db_mode_list.txt) 16 = head gear bottom 23 = looking dir
3 = max HP 10 = special AI state (?) 17 = head gear middle 24 = killer state (1 or 0)
4 = master ID (aid of the master, summon) 11 = SC option 18 = head gear top 25 = callback flag
5 = map index 12 = sex 19 = cloth color 26 = no random walk (1 or 0)
6 = x 13 = class (Monster ID, Job ID) 20 = shield

Example(s):

//this will set all the mobdata in the @array variable. (@array[1] being level, @array[13] class etc)
getmobdata .GID,@array;

//set the max hp of our poring to 1000.
setmobdata .GID,3,1000;