The giant evil superbrain IT has taken control of Charles Wallace's mind. On the face of it, Charles looks perfectly normal, but the words coming out of his mouth are clearly not his own. IT has got inside him and is using him as its mouthpiece. That's why Charles extols the virtues of IT and tells Meg and Calvin to relax and give up their fight against what he calls the "boss" and "The happiest sadist".
But there's no way that Meg and Calvin are ever going to do this. On the contrary, they're determined to destroy IT's evil power and the control he's exercising over Charles Wallace and the land of Camazotz. At first, this seems like a pretty hopeless task, especially after Mrs. Whatsit says that she's unable to do anything to help.
But Meg hits upon the idea of using the power of love to free Charles from IT's captivity. She knows that IT's power is based purely on hate, so she figures that if she expresses her love for Charles, even though he's currently possessed by IT, then she can free him from its wicked clutches. And she does. Once Meg tells Charles how much she loves him, Charles is immediately released from IT's control and warmly embraces his sister.