What was the shadowy thing?

Evil is a pretty apt description. Though Meg can’t adequately explain it, it is “dark and dreadful” and it covers the light of the stars. Calvin wants it to go away because he feels it is evil, and soon they can no longer stare at this darkness. Mrs. Whatsit descends back into the valley and Meg asks whether the dark thing they saw was what her father is fighting.

what is apt mean?

Apt means appropriate or fitting.