Mrs. Whatsit gives Meg her love. Mrs. Which gives her information.
Meg is terrified to go back to Camazotz to try to save Charles Wallace. She knows that IT is a powerful force and doesn't believe that she has the power to fight it. However, she also knows she's the only person who might be able to save her brother.
Mrs. Whatsit says her gift to Meg is her love. She says, "My love always." She tells her never to forget.
Mrs. Which's gift is information. She tells Meg that she has a power that IT does not. The thing she has is her only weapon, and she has to find it for herself.
Ultimately, Meg realizes that the thing she has is love. She uses love to save Charles Wallace from IT. Love is the thing that IT does not have.