This is the 828th most frequent Serbian word.
њене
"Her" (possessive pronoun).
In this sentence, 'њене' modifies the object 'проблеме' (problems), indicating that the problems belong to a female individual referenced in the context.
На њене проблеме се фокусирала.
She focused on her problems.
Here, 'њене' modifies the noun 'књиге' (books), which is plural, showing possession by a female individual.
Видела сам њене књиге на столу.
I saw her books on the table.
In this instance, 'њене' is used with 'речи' (words), again showing possession and agreeing in gender and number.