Generally, poisons are substances that cause death, injury or harm to organs, usually by chemical reactions or other activity on the molecular scales, when an organism absorbs a sufficient quantity.
Ingame, your characters can get poisoned through enemy attacks/spells or traps.
Unlike other afflictions, multiple doses of the same poison “stack”, meaning that successive doses combine to increase the poison’s DC and duration.
Making your initial saving throw against a poison means stacking does not occur—the poison did not affect you and any later doses are treated independently. Likewise, if a poison has been cured or run its course (by you either making the saves or outlasting the poison’s duration), stacking does not occur. However, if there is still poison active in you when you are attacked with that type of poison again, and you fail your initial save against the new dose, the doses stack. This has two effects, which last until the poisons run their course.
When enemy hits you with poison attack the DC of poison is calculated as [10 + HD/2 + CON modifier].
Increased Duration: Increase the duration of the poison by 1/2 the amount listed in its frequency entry.
Increased DC: Increase the poison’s DC by +2.
These increases are cumulative (a third dose adds another 1/2 of the frequency to the duration and +2 to the DC, and so on). When affected by multiple doses of the same poison, you only make one saving throw at this higher DC when required by the frequency, rather than one saving throw against each dose of the poison.
Multiple doses do not alter the Cure condition of the Poison, and meeting that Cure condition ends all doses of the poison.
Poisons can be delayed or completely negated, using a Delay Poison spell and cured with a Neutralize Poison spell.