The primary meaning of Samuel Taylor Coleridge's "Christabel" is that things are not always as they seem and that evil can hide under the appearance of good.
Christabel goes out in the night to pray, and as she is kneeling beside a tree, something moans nearby. That something turns out to be someone, a lovely lady dressed in white, who seems to be suffering greatly. She is beautiful, and Christabel immediately feels sorry for her and desires to help in whatever way she can. She takes the lady home with her, listening as they go to her tale of woe. Geraldine, as the lady calls herself, tells a story of kidnapping.
Yet Geraldine is not at all what she appears to be. Our first hint of that is when she has difficulty crossing the threshold of the gate. Geraldine seems to struggle in pain and sinks down. This is an indication of possible evil, for evil things cannot cross a threshold unless they are invited in. Geraldine cannot cross until Christabel lifts her over the threshold. Then she can walk easily again. Our next hint of something amiss is the dog's barking and howling in her sleep.
The two ladies wake the next morning and go to Christabel's father, Sir Leoline, to tell him Geraldine's story. Christabel begins to feel that there is something very wrong. She is frightened, yet she does not know why. Geraldine then looks "askance" at Christabel, and her eyes look like serpent's eyes, filled with malice. Christabel falls at her father's feet and begs him to send Geraldine away. She can say no more, having fallen under some kind of spell, and Sir Leoline becomes angry, believing that Christabel is dishonoring him and his hospitality.
This fragment of a poem ends before we discover what happens, but we can say for sure that Geraldine is something evil disguised as something beautiful and good and that things, indeed, are not always as they seem.