Vulnerability is the opposite of Resistance. When you have vulnerability to a damage type, you automatically take the maximum possible damage from any effect of that damage type - treat the attack as if it rolled the maximum possible result on each damage die when applying your damage only. Additionally, the attack is automatically considered a critical hit.
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. If you would gain vulnerability to a damage type you have immunity to, you are considered to have resistance to that damage type instead.