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The DC for creature abilities other than spellcasting is often 10 + half Hit Dice + ability score modifier + racial bonuses. The text you mention tells the GM which ability score was used to calculate the DC. This becomes important if the creature's ability scores change, such as by adding a template or rebuilding it more extensively. edit: I'm not sure if ability score damage also changes DCs tied to that ability score.
The DC for making a save against a monster's special ability (such as a spell-like ability or poison/venom) is determined by a fairly simple formula. It is 10 + 1/2 the monster's Hit Dice + an ability score modifier related to the ability.
For instance, if the creature has poison, the potency of its poison would be dictated by how hardy the creature is, thus its Constitution score. Let's assume the creature has a Con of 15 (+2 modifier) and 4 Hit Dice. The DC to save against the poison would be 10 + 1/2 HD (2) + Con mod (2) = 14. A character has to get a total 14 on his Fort save to avoid damage from the poison.
The reason they tell you what ability score affects the ability is so you can adjust it if the particular ability score goes up or down. For instance, if you advance the creature it might go up, and if you lower its scores with a spell that weakens it, the DC might have to go down.
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hey guys i was just wondering what the "DC is (ablity score) baised" in monster stats means and what used for? also how to use it?
Using the Dire Rat (Bestiary 232) as an example, the determination that the Filth Fever DC is Constitution based allows the GM to know how to scale that DC if the rat is affected by a Con damaging poison, or is buffed by Bear's Endurance, or if he used the size advancement rules in the back of the book (which would give it a +4 Con, and, therefore, a +2 bonus to the save DC of its disease).
It's also useful for a Druid who takes a Dire Rat as a companion, as the Con score is modified at 4th level, and could be modified by the druid through HD advancement or spells, to up the save DC of his rats disease effect.
The same guidelines would apply to any other creatures with a stat-dependent effect (such as a dragon's breath weapon, based off of Con) or a Barbed Devil's Fear effect or Spell-Like Abilities, which are based off of it's Charisma, and would be harder to resist if the devil was buffed by an Eagle's Splendor spell).