Stacked resists are resists OVER your maximum resist, to counter resistance-lowering effects such as: Conviction/Lower Resist/Cold Mastery.

Example of stacked resistance: Your Lightning Resist is at the cap of 75% in normal, but you have 50% in Hell. That would mean your TOTAL Lightning Resist is 150%, but your EFFECTIVE Resist amount is 75%.

Now, if someone use a Lower Resist curse on you, and you lost say... 60 to all resists, you would still be at 75%, since it subtracts from your TOTAL resistance, rather than your effective resistance.

Hope that helped. Feel free to ask about anything specific on that...