。
She was washing the dishes at the time.
Literal
She [topic-は] at-that-time dishes [object-を] was-washing.
皿を洗う ('wash dishes') is the everyday phrase for the chore — note the singular 皿 here even though the activity covers a stack: Japanese nouns are number-neutral by default, with quantity left to context or explicit counters. 皿洗い is the noun form ('dish-washing') that doubles as a job title. The polite ~ました frames this as a polite-spoken or written account, perhaps as part of an alibi or recounting of past events.