From PSP2i Wiki

Creatures can have Attack Resistances to an attack type. When a creature is hit with an attack type it resists, the damage it takes is halved.

There are three types of resistances: Strike Resistance, Ranged Resistance, and Technic Resistance. Each creature and their resistances can be found on Regular creatures.

If the player has Partner characters with them when encountering creatures with attack resistance, these partner characters will often indicate which creatures has resistance, as well as the type of attack they are resistant to.