Charles Wallace and Meg run into Calvin in the woods near Mrs. Whatsit's house as they are on the way to visit her. Charles Wallace is highly suspicious of what Calvin is doing there. He doesn't know Calvin, and he questions him about why he is walking in this particular spot.

Calvin finally explains that he came because he had a "compulsion" to be there. It was gut feeling, an intuitive knowing. Charles Wallace had, just before they saw Calvin, been describing to Meg his own intuitive sensibility. Charles Wallace tells Meg he just knows things about her without her having to say anything.

Calvin speaks about his own intuitive sense:

When I get this feeling, this compulsion, I always do what it tells me. I can’t explain where it comes from or how I get it, and it doesn’t happen very often. But I obey it.

Charles Wallace recognizes that Calvin is a kindred spirit. He realizes, too, that their intuitive sensibilities set them apart from Meg. She doesn't have "compulsions" or "knowings" in the same way they do. As Charles Wallace says:

Meg has it tough. . . . She’s not really one thing or the other.

In other words, Meg isn't simply an ordinary person like their twin brothers, but she also isn't intuitive like Charles Wallace or Calvin.