To add flavor or tenderize a food, it is often immersed in a seasoned liquid for a period of time. It is usually covered and refrigerated during the process. Most marinades are made with acidic ingredients (citrus juice, vinegar, wine, etc.) because the low pH helps break down foods.
The acids can also react with the container, so foods should be marinated in non-reactive dish such as ceramic, glass or stainless steel or in a plastic bag.