How did Charles Wallace know that the tesseract thing was something big to their mother?

Charles Wallace sensed that the tesseract was significant to their mother, Mrs. Murry, because of her deep emotional reaction and the way she spoke about it. He could see her passion and concern when discussing her research, indicating that it held great importance for her. Additionally, his intuitive understanding of her thoughts and feelings allowed him to grasp the weight of the tesseract's implications for their family and the universe. This connection underscored the bond between them and highlighted the tesseract's potential impact on their lives.