Meg, Calvin, and Charles Wallace are taken to the Happy Medium so that she can show them the Black Thing which covers all or parts of some planets in the universe. The Black Thing is the book's term for evil.
The three children see that the Black Thing partially covers the planet earth. This is distressing to them, but they learn that evil is kept from taking over the earth by the great religious leaders, thinkers, artists, and scientists who work to keep it at bay. The children are then shown Camazotz, the planet where their father is being held. This planet is entirely covered in darkness because the Black Thing has entirely taken over there.
The Happy Medium can offer the children a clear visual view of the Black Thing. This view helps the children understand what they will be up against as they travel to Camazotz to attempt to free Mr. Murry.
It is troubling to the three young travelers to see a visual representation of evil and to know it is loose in the universe, but Mrs. Whatsit, Mrs. Which, and Mrs. Who also try to instill the children with hope that this darkness can be defeated as long as people like them take on the fight.