AtD translation: For something to actually . . . light up

[Mike Jing]

P499.3-7   Cyprian, it’s nothing I expected. We are sent here, most of us,
aren’t we really, to stay out of the way, not be a bother—the books, the
tutoring, the learning, it’s all incidental.  For something to actually . .
. light up, it’s . . . no one would believe me, if I . . . oh, one or two
boys in Hardy’s classes, but certainly no one back at Chunxton Crescent.

What does "light up" mean here exactly? And what was left unsaid after
"it's . . ." and "if I . . . "?