Translation:Do you understand?
At least one source I've seen indicates that 不明白 and 不懂 are interchangeable.
"不懂 (bù dǒng) may also be swapped out with 不明白 (bù míngbai)." Source: https://resources.allsetlearning.com/chinese/grammar/Potential_complement_%22-bu_dong%22_for_not_understanding
I think 明白 literally means "(the situation) is clear to me, cause that would make sense; 明 means bright after all, and I think together with 白, they mean bright and clear. "understand" as one word would be 懂, but I remember reading somewhere that it means more like understanding what the person said, the words they used. I imagine 明白, or more specifically “你明白了吗？” being used after a long (or short) explanation of the situation one's in.