The spell captures some of the incoming energy associated with the outer planes, lessening its effect on you and storing it as arcane energy throughout your body. You have resistance to the triggering damage type and gain a temporary +2 to your AC until the start of your next turn. 

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the temporary increase to your AC goes up by 1 for each slot level above 1st.