This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
瓐 is a rare character whose historical origin is uncertain. It visually contains 盧 on the right, but the exact development is unclear.
Since the meaning is unclear, focus on the shape: 王 (jewel) on the left and 盧 on the right. Imagine a rare, uncut gemstone with a rough, complex surface like the strokes of 盧.
For ロ, picture a low, rumbling sound coming from the mysterious gem: low -> ロ.