The stained glass window is an odd symbol in the sense that it doesn't actually appear in the novel itself: David vaguely remembers seeing such a window sometime in his early childhood, but can't exactly place the image. The memory resurfaces, however, when he first meets Agnes Wickfield, and from that moment on, it repeatedly appears in connection with her. The image of beautiful, soft light filtered through a window symbolizes Agnes's gentle kindness and calming demeanor, while the religious overtones of the window (David associates it with a church) hint at her "angelic" nature, as David frequently refers to Agnes as his “good Angel.”
Stained Glass Window Quotes in David Copperfield
I cannot call to mind where or when, in my childhood, I had seen a stained glass window in a church. Nor do I recollect its subject. But I know that when I saw her turn round, in the grave light of the old staircase, and wait for us, above, I thought of that window; and that I associated something of its tranquil brightness with Agnes Wickfield ever afterwards.