。
She doesn't want to talk about it, you know.
Literal
She [topic-は] that thing [about-について] talk-want-not [explanatory-んだ] [emphasis-よ].
について ('about, regarding, concerning') is one of the most useful composite particles. Built from the verb つく ('attach') in te-form plus に, it literally means 'attached to' — i.e. 'on the topic of.' The ending んだよ combines the explanatory んだ with the assertive よ — 'the situation is that she doesn't want to talk, just so you know.'