Keller writes that “Light! Give me light” was the wordless cry of [her] soul”. What was the ”light” Keller longed for, and how did receiving it change her life?

From the beginning of Keller's education, Anne "Annie" Sullivan was convinced that there was a great torment in Keller's soul and that the torment came from being fundamentally unable to communicate with the world around her. Worse yet, Annie observed that Keller's parents seemed far less concerned with actually helping her find her way out of this darkness and more with simply having her trained enough to not be a menace in day-to-day home living. This turned out to indeed be true, and the "light" that Keller was referring to was language. Language allowed Keller to communicate with the world around her and to understand and contextualize its meaning as it related to her. After that fateful afternoon by the water-pump, where Keller learned her first word, she was able to feel less like a lost child alone in a dark, silent world. She understood the people around her cared for her and that she was a part of the world of the living.

As she explains in her memoir, the "light" Helen Keller needed was a way to communicate with the rest of the world. As she grew older and her mind became more sophisticated, she became more and more frustrated at her inability to express herself, to learn, or to relate to people beyond a primitive level. She was so frustrated by age seven that she was having daily tantrums. It was as if she were kept in a box growing tighter and more constricting with every passing day.
The "light" she needed came in the form of Anne Sullivan. Miss Sullivan's arrival changed everything for Keller, and she can hardly find words to describe how profoundly life-giving her teacher's coming was. Keller likens it to the miracle of God parting of the Red Sea so that the Israelites could pass from bondage to freedom.
Miss Sullivan frees Helen by teaching her how to communicate through writing in her hand. This breakthrough changes Helen's entire life and opens the world up to her. Miss Sullivan also gives Keller the love, friendship, and guidance she craved.

Helen's describing that fateful afternoon when Annie Sullivan gave her the gift of language, allowing Helen to begin her education in earnest. For the first time in her life, Helen now knows that there's a way forward. This is the "light" to which she's referring, a bright path that cuts through the darkness of a world previously incomprehensible to her.
Despite her various challenges, Helen had always had a passionate commitment to the acquisition of knowledge, yearning for an education that would do justice to her immense intellectual curiosity. But before Annie came into her life, Helen felt like a lost ship at sea, trapped by an endless fog, hoping against hope for something to happen.
On the most important day in Helen's early life, Annie takes Helen's hand and places it under a water spout. As Helen feels the cool, insistent gush against her hand, Annie takes her other one and spells out the word "water" using her fingers. As Helen describes it herself, that one little world awakened her soul, gave it light and hope, revealing to her for the very first time the mystery of language. Now Helen can begin her education at last; now she has light. From this day forward, her life will never be the same again.