Wisdom

Wisdom (Wis) describes a composite of the character's enlightenment, judgment, guile, willpower, common sense, and intuition. It can affect the character's resistance to magical attack. It is the prime requisite of priests; those with a Wisdom score of 16 or higher gain a 10% bonus to experience points earned. Clerics and druids with Wisdom scores of 13 or higher also gain bonus spells over and above the number they are normally allowed to use.

Explanations
Magical Defense Adjustment applies to saving throws against magical spells that attack the mind: beguiling, charm, fear, hypnosis, illusions, possession, suggestion, etc. These bonuses and penalties are applied automatically, without any conscious effort from the character.

Bonus Spells indicates the number of additional spells a priest (and only a priest) is entitled to because of their extreme Wisdom. Note that these spells are available only when the priest is entitled to spells of the appropriate level. Bonus spells are cumulative, so a priest with a Wisdom of 15 is entitled to two 1st-level bonus spells and one 2nd-level bonus spell.

Chance of Spell Failure states the percentage chance that any particular spell fails when cast. Priests with low Wisdom scores run the risk of having their spells fizzle. Roll percentile dice every time the priest casts a spell; if the number rolled is less than or equal to the listed chance for spell failure, the spell is expended with absolutely no effect whatsoever. Note that priests with Wisdom scores of 13 or higher don't need to worry about their spells failing.