This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 昈 is uncertain. It visually combines 日 (sun) and 户 (door), but the original purpose of this combination is unclear.
The sun 日 beside a door 户 might suggest light coming through a doorway, but since the meaning is uncertain, treat this as a visual guess only.
For コ, imagine a small child (子, こ) peeking through the door 户 at the sun 日: 子 -> コ, and the child sees the bright light.