This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
皨 is a rare character whose exact historical development is uncertain. It visually combines 皛 (a compound of 白 and 日) with 土 below, but the original semantic or phonetic rationale is unclear.
The top part 皛 suggests brightness (white + sun), and the bottom 土 is earth. Picture bright light shining on the earth, but the exact meaning is unclear.
For セイ, imagine a bright ray of light (say) hitting the earth: say -> セイ, and the light illuminates the ground.