What is the writing in the margin of a text called that better helps you understand the passage?

Some one drawing water and my theater placed my hand under the spout. as the cool stream gushed over one hand she spelled into the other the word water , first slowly , then rapidly . i stood still ,my whole attention fixed upon the motions of her fingers . suddenly i felt misty consciousness as of soothing forgotten --a thrill of returning thought and somehow the mystery of language was revealed to me this passage from the stoyof my life is an wxample of which element of plotclimax action