What two things did Mrs. Whatsit do that concerned Meg and Charles Wallace?

When the children are going to Camazotz the first time, Mrs. Whatsit gives Meg the gift of her faults as her protection and offers Charles Wallace the gift of his resilience.
Later, when Meg returns to Camazotz on her own to find and rescue Charles Wallace from the grip of IT, Mrs. Whatsit sends Meg off her with her love.
Focusing on what Mrs. Whatsit has given her allows Meg to use her faults—her anger and stubbornness—to withstand the assaults of IT. When she is rescuing Charles Wallace, Meg focuses on love. She suddenly realizes that love is the one feeling she has for her brother that IT can't offer. She is able to use her love to get her brother back.
In a more general sense, Mrs. Whatsit helps all three children tesser to Camazotz to find Mr. Murry. She shows both Meg and Charles Wallace a great deal of love.