After many strange events and adventures with the alien Mrs Ws, Meg has discovered that the universe is threatened by a cloud of evil called "The Black Thing." Worse, her little brother Charles allowed himself to be corrupted in order to find their father. Meg is nearly killed, and the Mrs Ws cannot transport Charles while he is under the control of IT, an evil telepathic brain. Despite their power, the Mrs Ws cannot help Charles, but they tell Meg that she can, with a power under her control that can fight and defeat IT.
If she could give her love to IT perhaps it would shrivel up and die, for she was sure that IT could not withstand love. But she, in all her weakness and foolishness and baseness and nothingness, was incapable of loving IT.
(L'Engle, A Wrinkle in Time, Google Books)
Meg is close to slipping into IT's control, and she is sure that she cannot try to defeat IT with love, because she cannot love IT. However, she realizes that she can love Charles, despite IT's control of him; she focuses her love on Charles, ignoring everything else around her, and breaks him free of ITs mind control. With Charles free, the Mrs Ws are able to transport them back to Earth, where they are reunited with their father and family.