Damage Resistance

Resistance to a damage type means damage of that type is reduced against you.

Halved damage. When you take damage of a type you have Resistance to, that damage is halved against you (round down). Example: 13 Cold damage becomes 6 if you have Resistance to Cold.
No stacking. Multiple instances of Resistance to the same damage type count as only one. Having Resistance to Necrotic and Resistance to all damage still only halves Necrotic damage once.
Order of application. Damage modifiers apply in a set order: first any bonuses, penalties, or multipliers; then Resistance (halving); then Vulnerability (doubling). So an outside effect that reduces incoming damage is subtracted before your Resistance halves what remains.
Applies per instance. Resistance affects each instance of qualifying damage as it is dealt, not your total damage for the turn.

Resistance is not the same as Immunity (no damage) or Vulnerability (doubled damage).