Intelligence determines how well your character learns and reasons. This ability is important for wizards because it affects how many spells they can cast, how hard their spells are to resist, and how powerful their spells can be. It’s also important for any character who wants to have a wide assortment of skills.
You apply your character’s Intelligence modifier to: -The number of languages your character knows at the start of the game.
-The number of skill points gained each level. (But your character always gets at least 1 skill point per level.)
-Appraise, Craft, Decipher Script, Disable Device, Forgery, Knowledge, Search, and Spellcraft checks. These are the skills that have Intelligence as their key ability.
-A Wizard and Wu Jen all gain bonus spells based on her Intelligence score. The minimum Intelligence score needed to cast a wizard or Wu Jen spell is 10 + the spell’s level.
An animal has an Intelligence score of 1 or 2. A creature of humanlike intelligence has a score of at least 3.
This is a primary stat for Wizard and Wu Jen who should keep this at the highest possible, an 18 is prefered if available. Classes based on doing skills such as the bard, rogue, or factotum all need a high Intelligence score as well, shooting for around a 12 at minimum. Other classes can, but usually should not drop Intelligence.
Keep in mind that the minimum Intelligence needed to be a playable character is a 3, meaning if your race gets a huge int penalty, and you roll poorly put your worst roll into intelligence. A character always gains 1 skill point per level, and has a minimum of 3 Intelligence. This means if you get a racial -4 to Intelligence, and roll a 6, for it. You will have an Int score of 3, which mechanically isn't any worse than an Intelligence score of 9.
For more uses of Intelligence see the X Stat to Y Bonus