。
She did the dishes voluntarily.
Literal
She [topic-は] [voluntarily-自発的に] dish-washing [object-を] did.
自発的 (じはつてき, 'spontaneous, voluntary') is a な-adjective; in adverbial form 自発的に it modifies the verb. The 自発 root literally combines 'self' (自) with 'emit/initiate' (発) — acting on one's own initiative, not from external pressure. 皿洗い is a verb-stem-plus-noun compound — 皿 'plate, dish' + 洗い (renyōkei of 洗う 'to wash') — meaning 'dish-washing' as a noun. The construction noun + を + する is hugely productive in Japanese for turning activities into verbal phrases.