。
She was washing dishes.
Literal
She [topic-は] dish-washing [object-を] was-doing.
皿洗い ('dishwashing') is built from 皿 ('plate, dish') + 洗い (the masu-stem of 洗う 'wash'). The whole noun phrase becomes the object of する with を. The progressive ~ていた marks the dishwashing as ongoing in the past — 'was washing.' Compound nouns of [object]+[verb-stem] are productive across many domains: 雪かき ('snow-shoveling'), 草刈り ('grass-cutting'), 皿洗い ('dishwashing'), 床磨き ('floor-polishing').