。
She was wearing a red dress.
Literal
She [topic-は] red dress [object-を] was-wearing.
ドレス is a loanword from English 'dress,' typically referring to a more formal garment — an evening dress or party dress — rather than everyday clothing. Japanese has a rich set of clothing vocabulary mixing native words (服, 着物, 浴衣) with numerous loanwords (ドレス, コート, ブラウス, スカート).