。
I could understand what she was saying.
Literal
I [target-には] she [genitive-の] saying thing [subject-が] understood.
Subtle use of には. The topic is marked には ('as for me, to me') rather than just は — this nuance marks 'I' as the one experiencing the understanding, different from agency. The subject 彼女の言うこと ('what she says') uses が because it's the subject of 分かる, which takes が-marked objects. A classic example of the Japanese [experiencer]には + [stimulus]が pattern.