Immunities, Resistances, and Weaknesses of creatures indicate how effects and damage interact with that creature. Immunities are effects and damage types which a creature is unaffected by, while resistances are effects and damage against which a creature is not fully affected by. Weaknesses are effects and damage against which a creature is more susceptible. The following lists the particular details of each.
Immunities have a type. Immunities will always list a tag and you ignore all damage that has that tag.
Resistances lessen the amount of damage you take of a particular type. Resistance will always list a tag and value. Every time you take damage with that tag, subtract the resistance value from the tagged damage you are dealt before you apply it to your hit points. Some resistances are against all damage.
Weaknesses also have a type. Weaknesses will always list a tag and you take double damage that has that tag.