If a restaurant served you something you didn’t order and charged you for it, would you pay? In Japan, you don’t always have a choice.
In many 居酒屋 (izakaya, or pub), ordering even one drink often subjects you to a small, appetizer-like dish called お通し (otooshi). What you get varies: cooked vegetables, potato salad, and bean sprouts and pork are all examples of お通し.
While the dish changes from day to day, location to location, the fact that it costs around 300円 does not. Some places might allow you to skip the お通し, while others treat it like a table charge (i.e. non-negotiable). Regardless, don’t be surprised if you’re served one of these dishes — though if you don’t want it, don’t be afraid to ask if you can pass.