。
She works at a trading company.
Literal
She [topic-は] trading-company [at-に] is-working.
勤めている describes an ongoing employment state using ~ている. 勤める takes the particle に to mark the workplace, and carries a nuance of holding a regular salaried position — more specific than the general 働く (to work). 商事会社 refers to a company engaged in trading and commerce. Japan's large 総合商社 (general trading companies) are a distinctive feature of the Japanese economy, handling everything from raw materials to technology imports.