。
She was wearing a simple dress.
Literal
She [topic-は] simple dress [object-を] was-wearing.
簡素 is a na-adjective meaning 'simple, plain, austere' — distinct from 簡単 ('easy'), which shares the first kanji 簡 ('brief / simple'). Where 簡単 emphasizes ease of doing, 簡素 emphasizes lack of ornamentation: a 簡素なドレス is a dress with clean lines and no fuss, slightly literary in flavor. ~ていた ('was wearing') uses the past form of ~ている in its 'state' sense — 着る ('put on') is a momentary verb of donning, and ~ている turns it into the lasting state 'be wearing,' so ~ていた describes 'was in the state of wearing.' This stative-past pattern is essential for clothing in Japanese narrative.