。
I can't understand what she said.
Literal
I [topic-は] her [possessive-の] said [thing-こと] [subject-が] understand-can-not-[insistence-よ].
彼女の言った is a relative clause modifying こと — note the の instead of が in the relative clause (the の-for-が substitution pattern). 言った is past tense. 理解 ('understanding') + できる ('can do') in the potential sense. A frustration-tinged statement.