This is the 406th most frequent Estonian word.
milles
"In what" or "in which."
In this sentence, 'milles' is used similarly to mean 'in what,' asking about the aspect of importance of a topic.
Palun selgitage mulle, milles on selle teema olulisus.
Please explain to me the importance of this topic.
In this question, 'milles' refers to 'in what' or 'what does it consist of,' asking for detail about the problem's cause.
Milles seisneb probleemi põhjus?
What is the cause of the problem?
Here, 'milles' means 'in what,' referring to the specific aspect being doubted in the statement.