Why was Mrs Murray so alarmed when Mrs Whatsit mentioned a tesseract?

Mrs. Murray was alarmed when Mrs. Whatsit mentioned a tesseract because it suggested a deeper understanding of complex concepts that could involve danger. The term "tesseract" implies a manipulation of space and time, which is beyond her comprehension and suggests that her children might be involved in something perilous. Her concern stems from a protective instinct as a mother, fearing for the safety of her family amid unknown cosmic forces.