Stacking is the concept where more than one instance of the same type of effect is cumulative. The rules that determine if something can stack or not are very unclear, and there are many exceptions to the rules. Most of these take into consideration where the effect comes from (e.g., gaining a dodge bonus from a feat will stack with one from an item, but you cannot stack 2 from different items).
Examples of things which do stack:
- Ability score bonuses stack up to +12.
- AC dodge bonuses stack up to +20.
- Saving throw bonuses stack up to +20.
- Skill bonuses stack up to +50.
Examples of things which do not stack:
- Bonuses to your AC of the same type (except dodge) from different items such as the deflection bonuses from a Ring of Protection and a Cloak of Protection.
- Multiple uses of the same spell such as Bull's Strength (but the spell does stack with the feat Bull's Strength).