What does Calvin mean when he describes the planet earth as one that is shadowed, but fighting the shadow?

Calvin specifically means that although evil shadows their world, there are people on the earth actively fighting against that evil.

Gradesaver addresses this in their theme page for the novel;

L'Engle's novel also gives a compelling argument for a characterization of evil that fits into a Christian framework. While several of L'Engle's characters, including the Man with the Red Eyes, embody elements of evil, the element that most takes on the form of greatest evil is the Dark Thing.

The Dark Thing, a mass of darkness that fills the universe blocking the light of suns and stars, is based on a particular Christian theological notion of sin. While sin is often characterized in Christianity as a particular act of evil - such as murder - it can also be understood to be a general force in the universe, a darkness, that is combated by forces of good, usually represented by light.

This conception of evil is not necessarily a specific act or instance, then, but instead an invisible force that is the root cause of the evil in the world. This is demonstrated best in the novel when the Medium shows the children the planet Earth and suggests that all of the evil and confusion in the world is really a result of the Dark Thing that surrounds the Earth's atmosphere. Through the Dark Thing, L'Engle is able to personify sin into a defeatable force that the children can physically combat.

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