Resistance is a self-explanatory term. It simply refers to the character or creature’s natural or magical resistance to a particular type of aggression and its natural defenses and resilience. If you have Resistance to a specific kind of attack, you only take half the allotted damage from it, rounded down.
Alongside Resistance, there is also Immunity and Vulnerability, which should also be explained. Immunity to something means you take no damage from that type of attack. Vulnerability means that you are so susceptible to something that you take double damage from it.
Some natural Resistance will stack with other forms of damage reduction. The Rogue’s Uncanny Dodge ability would stack perfectly with Resistance meaning that only a quarter of the damage total is applied.
Similarly, an ability such as Stone’s Endurance stacks with Resistance, meaning the damage is reduced and then halved. Resistance is always applied after any other stacked ability or consideration.
How to Get Resistance
Some races come with natural Resistance built in. Dwarves (Poison), Tiefling (fire), Aasimar (radiant and necrotic), and Dragonborn (dependent on their ancestry) are born with natural Resistance to the type of damage types listed in their racial traits.
Certain spells also grant Resistance. These include. Protection from Poison (Poison), Absorb Elements (varies), Stoneskin (non-magical Bludgeoning, piercing, and slashing), and Protection from Energy (varies). Many others are available; a quick check with the Abjuration spell lists will give you all the options available.
Some spells, such as Mind Blank, will confer Immunity in certain situations, though the price is often high.
Magic Items are obviously a great source of magical protection through Resistance. Examples include Armour of Resistance (varies), Brooch of Shielding (force), Potion of Resistance (varies), Ring of Warmth (cold), and Absorbing Tattoo (varies).
And as with spells, more powerful items, such as Armor of Invulnerability, grants complete Immunity in certain situations.
Of course, these are just some examples of how Resistance works from the core rules. Nothing can stop you and your DM from getting together to create other systems, items, and options.
Resistance is a potent ability, whether it comes from racial genetics, spell casting, or possession of a magical item. It is often the difference between that ogre’s club bouncing off of your armor for minimal damage and getting knocked into a different county. Cherish it and use it well.