This is the 800th most frequent Catalan word.
quins
"Quins" means "which" or "what" (plural form).
'Quins' is functioning as an interrogative adjective, asking about specific days.
Quins dies fa sol aquesta setmana?
What days is it sunny this week?
'Quins' specifies the books among a possible selection.
No sabia quins llibres voldries llegir.
I didn't know which books you would want to read.
'Quins' expresses an indefinite but specific idea of problems in context.