As a Dark Elf, I have a racial bonus of 15% fire resistance. Does this mean, I evade the damage in 15% of the cases, or I only take 85% thereof? Furthermore, I have a fire resistance of 24 bestowed by my boots.
With resistances as a percentage, incoming damage is reduced by that percentage - i.e if you were to take 100 fire damage with 15% resistance, you would take 85 fire damage instead.
Resistances as a number are simple flat mitigations - 81 resistance to frost damage means all incoming frost damage is reduced by 81.
Flat resistances are applied before percentile, so a fire resistance of 24 and 15% against 100 damage leads to 76 then 64.6 total damage (which I believe is rounded up to 65).
All bonuses stack, even if the GUI claims otherwise: the GUI only displays total flat changes when it shows a number next to damage or resistance; percentage changes are not shown on the GUI. However, they are actually added in practice. It's not the best GUI design, but that's how it works.
It means that you take 85% of the damage. Resistance means to resist and henceforth if it would have been 15% chance of not getting damaged it would quite frankly be overpowered and it would be mentioned in a way like evading or etc.
Also about the potions if freeze does 100 damage you would take 19 damage for 10sec. It subtracts the damage you would take from the damage inflicted.