used to refer to a space used for storing things that is large enough for a person to enter and walk around in

a walk-in wardrobe 步入式衣橱，衣帽间

a walk-in refrigerator 步入式冰箱

used to refer to a place that you can go to without an appointment (= an arrangement made in advance)

It's a walk-in dental clinic. 这是一家无需提前预约的牙科诊所。