You cast spells that more easily penetrate spell resistance.
Benefit: A fortified spell is treated as having a higher caster level for the purpose of defeating a target’s spell resistance. You prepare and cast the spell in a higher-level spell slot than normal, with each additional level giving a +2 bonus on spell penetration checks for the altered spell. Spells that are not subject to spell resistance are not affected.
A fortified spell uses up a spell slot at least one level higher than the spell’s actual level.
Source: Complete Arcane, page 80.