Resistance is the opposite of vulnerability, and a weaker form of immunity. When you have resistance to a damage type, you reduce all damage of that damage type by half. If the effect achieves a critical hit or inflicts a condition, ignore the critical hit or condition, but also ignore your resistance - the two effectively cancel out.
If two effects would grant you vulnerability and resistance to a damage type, the vulnerability and resistance cancel out - treat all damage of that type as normal. On the other hand, if two different effects each grant you resistance to the same damage type, they do not combine into immunity - you merely continue to have resistance.