This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 晿 is uncertain. It visually combines 日 (sun) and 昌 (prosperous), but its original meaning and component function are unclear.
Since the meaning is uncertain, focus on the shape: 日 (sun) beside 昌 (prosperous) might suggest a bright, flourishing sun, but this is only a visual mnemonic, not a confirmed meaning.
For ショウ, imagine a show where the sun 日 shines brightly on a prosperous 昌 stage: show -> ショウ.