Resistance to something halves damage.
Vulnerability to something doubles damage.
Resistance and Vulnerability are applied last, so all damage is calculated and then resistances or vulnerabilities are applied to the final tally.
Resistances and Vulnerabilities don't stack. E.g. if you're resistant to fire and bludgeoning, and somebody hits you with a hammer made of fire, you only get one resistance bonus, it's not halved, then halved again.