Stacking is the concept where more than one instance of the same type of effect is cumulative. The rules which determine if something can stack or not is very unclear, and there are many exceptions to the rules. Most of these take into consideration where the effect comes from (i.e., 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 scores bonuses stack up to +12.
- AC dodge bonus stack up to +20.
- Saving throw stack up to +20.
- Skills stack up to +50.
Examples of things which do not stack: