This is the exposition in the story. We learn Salva is living in South Sudan in the middle of the 2nd civil war. His mind drifts off as he thinks about where he would rather be, at home, taking care of cattle. This event shows us how Salva normally is, which is why it's important.
The inciting incident in A Long Walk to Water is at the part where Salva and his classmates are running to "The Bush" to get away from the gunshots. This creates a "spark" in the story by moving him away from home and into the rising action setting.
Salva is running into The Bush but Northern Soldiers are there to stop them, he runs away while the men are used to fight back. It is building suspense because he now has only one option, run with the women and children and hope to find shelter.