What is Add Round Key

Question

What is meant by the term Add Round Key.

Answer 1

In Add Round Key, 128 bits of the State include bit wise XORed with 128 bits of round key. The operation is considered as the column wise operation between 4 bytes of the State column and one word of round key; it may also be viewed as the byte-level operation. The Add Round Key transformation is as simple as possible and affects each bit of the State.