In chapter 5 why do the children have to go behind the shadow

Mrs. Which then tells them that it is time to “go behind the shadow” and begins to explain how they travel through space and time. Mrs. Whatsit explains the meaning of the tesseract. Using some illustrations and diagrams, Mrs. Whatsit shows them how they “wrinkle” time and space, making travel between great distances easy and quick. The tesseract is the fifth dimension of space, a dimension the children can travel through meaning that “a straight line is not the shortest distance between two points.” It was a concept Meg’s parents have been working on for a tremendous amount of time. Meg understands the concept for a brief second before losing her comprehension. Charles Wallace, meanwhile, seems to understand it completely.