This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
坢 combines 土 (earth) with 半, which likely contributes a phonetic element. The exact historical development is uncertain.
The left side is earth 土, and the right side is half 半. Picture half of the earth being flat and smooth, like a leveled plain.
For ハン, imagine a hand smoothing half the earth: hand -> ハン, and the flat earth is half done.