The Happy Medium wants to cheer up the children after showing them the Dark Thing and Camazotz. However, her plan backfires. She reveals Calvin's unhappy home life in her crystal, and then she shows Meg her mother. Instead of making her happy, the vision makes Meg angry, because her mother is huddled up sadly in her lab at home. She has dropped her head to the paper she was writing on, and her unhappiness is apparent. This is a side of herself she never lets her children see.
Meg, who has been protective of those she loves throughout the book, again wants to jump into action, this time to help her mother. Meg's response to her mother's sadness is the following:
"Let's go!" she cried harshly. "Let's do something!"
Meg's anger gets her into trouble, but it also motivates her to take positive action to help people, which is why Mrs. Whatsit tells her to hang on to it.
Meg becomes angry when the Medium shows her a vision of her mother at home, writing a letter to her absent husband, sinking into a depth of unhappiness that she does not allow her children to see. This gives Meg a larger perspective on the power of evil, not just that it has captured her father. She sees that, an attack on one person affects all those who love that person, and thus continues on, spreading like a cancer.
Meg's anger is righteous anger, directed at evil and the consequences of evil. This spurs her to action, willingly going to Camazotz to free her father. This anger allows her to push past her fear and her self-doubt. Her focus ceases to be on herself but on her father.
Mrs Whatsit's parting words to Meg are, "Stay angry, Meg. You will need all your anger." In stating this, Mrs. Whatsit ascertains the nature of Meg's anger and encourages her to retain it as a means to focus her energy, especially as she encounters the nature of evil itself.