an ambulance救急車 means “ambulance,” and を marks it as the object of a verb. This phrase is usually followed by a verb such as 呼ぶ (to call), as in 救急車を呼ぶ (to call an ambulance).
JA: 事故を見たら、すぐに救急車を呼んでください。
EN: If you see an accident, please call an ambulance immediately.