Why was mrs. murry so alarmed when mrs. whatsit mentioned a ''tesseract''?

Mrs. Murry was alarmed when Mrs. Whatsit mentioned a "tesseract" because it suggested a deeper understanding of the universe and dimensions beyond her comprehension, which could pose a significant danger. The term implied a complex concept of space and time that could potentially affect her family and their safety. Additionally, her concern was heightened by the mysterious nature of Mrs. Whatsit and the implications of traveling through dimensions.