This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
坨 combines 土 (earth) with 它, which likely contributes a phonetic element. The exact historical development is uncertain, and the character is rare in Japanese.
With 土 (earth) on the left and 它 on the right, 坨 might suggest a mound or lump of earth, but its meaning is unclear.
For タ, imagine a small mound of earth shaped like a tar pit: tar -> タ, and the earth component 土 ties it to the ground.