Translation:They do not understand.
The word you mentioned is 懂 dong (instead of 等 deng / wait).
明白 barely means to understand, while 懂 means to know or understand something to an extent you can manoeuvre it, deduce something out of it, or take proper action subsequently. If you say 我懂法語 then it is usually expected that you can read, write, speak and listen to French. When you say 我明白這句法語 you probably only understand the meaning of that particular French sentences and it does not imply that you would know other French sentence or speak French at all.
sounds more like "they do not remember" to me I usually use: "我记得“ to indicate I remember "我明白” to indicate I memorized "我知道“ for I know and "我懂” for I understand
Shouldn't the sentence "They don't understand." also be considered a correct one? Duo says it's a typo...