。
I wonder what she's worried about.
Literal
She [topic-は] what [object-を] is-worrying [I-wonder-のだろう].
~のだろう is the soft-musing variant of plain ~だろう, combining the explanatory のだ with the conjectural だろう. It's used for thinking aloud — 'I wonder what's behind this.' Its sister form ~のですか takes the same explanatory base and turns it into a question directed at a listener; ~のだろう keeps the explanatory flavor but stays inward. のだろう can be addressed inwardly to oneself or said aloud as a wondering not directed at any particular listener. A useful pattern for narrative musing.