You shape your ward to mimic an incoming attack in order to parry . You cast this spell before the enemy roll to hit is announced and declare the level of spell you wish to cast it at. After a confirmed hit you roll the same number of dice and dice type the enemy rolls for damage and reduce the damage equal to the number you rolled. 

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the number of attacks blocked against one creature go up equal to the spell level.