Spiritual Resistance is like Fire Resistance and Electrical Resistance. The higher your resistance, the less damage you will take from that element. Spiritual Resistance is like your will power. You are less likely to be knocked unconsious or fall foul of curses or mental attacks (from Ghosts and such).
Enemies also have these resistances. You need the Know Creature skill to find out what resistances they have, or you can experiment.
