Each Magic+ point increases the amount of extra damage that the PC does by 1 (as shown in the Damage field in the character info screen,) and increases the chance of hitting the opponent by 5% (i.e. a Mace +2 has +2 Damage and +10% Chance to Hit). For more info on 'Damage' and '% Chance to Hit' see the manual.
Notation:
Icon/Name/Scenario/Damage Vs. Small/Vs. Large/Weight