Player > Feats > Improved Energy Resistance
Your natural resistance to an energy type is more pronounced.
Con 13, character level 10th, energy resistance from a racial trait.
Your resistance to one type of energy damage from a racial trait stacks with one other source of energy resistance. If it already stacked, that resistance increases by 5 instead.
You can take this feat multiple times—its effects don’t stack. Each time you take this feat, it applies to a different energy resistance from a racial trait.