This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 晆 is uncertain. It combines 日 (sun/day) with 圭, but the original meaning and component function are unclear.
The sun 日 beside 圭 suggests a possible connection to daylight or a time of day, but without a clear meaning, treat it as a shape to recognize rather than a story to remember.
For カイ, imagine a bright sun 日 shining on a kai (shell) shaped like 圭: kai -> カイ, though the meaning remains uncertain.