Casting in Combat
When casting a spell during combat wizards and priests loose their DEX bonus to AC, since you must stand still and not be moving wildly around. If 1 point or more of damage is dealt the spell is disrupted and lost. The loss of DEX bonus to AC occurs only for the duration of casting, before casting starts (your base initiative) and after your finished casting (base initiative + casting time) DEX is applied.
Taking the Concentration NWP provides a chance to prevent disruption if the damage is 2 or less.