Resistances offer creatures and objects full or partial protection against certain kinds of attacks and damage. Either innate or temporary protection (potion of fire resistance). A fire elemental would take reduced damage - if any at all - from heat-based attacks. A piece of armor might be acid-proof or rust-proof. Symmetrically opposite to resistances are vulnerabilities, e.g. an ice giant might be weak against fire. Often, resistances will be based on the elements.