You can take this reaction when you are hit by an attack you can see or targeted by magic missile.
A shimmering barrier of magical force appears to protect you. Until the end of the current turn, you gain a bonus to your Armor Class equal to 1 + your Spellcasting modifier and you take no damage from the magic missile spell.
At Higher Levels. When you cast this spell using a spell slot of 2nd-level or higher, the bonus to your Armor Class increases by 1 for each slot level above 1st.
You touch a willing creature that is not wearing metal armor. For the duration, it's skin has a rough, bark-like appearance, and its Armor Class cannot be lower than 16. The spell ends if the target is Incapacitated or dons metal armor, but it can wield a shield and retain this benefit.