This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 兤 is uncertain. It visually combines 光 (light) outside and 廣 (broad) inside, but the original semantic or phonetic motivation is unclear.
With no clear meaning, picture light 光 spreading broadly 廣, but remember that the actual meaning is uncertain.
For コウ, use the common reading of 光 (こう) as a hook: light 光 gives コウ, even though the kanji's meaning is unclear.