。
Her laughter was a lie to hide her sadness.
Literal
She [topic-は] laughter [topic-は] sadness [object-を] hiding lie was.
A double-は construction: the first は marks 彼女 as the sentence topic, the second marks 笑い ('laughter') as the subtopic being characterized. The relative clause 悲しみを隠す ('that hides sadness') modifies うそ ('lie'). The sentence reads like a literary observation — her smiles were a mask for grief.