Mirin is a sweet sake for cooking, in this case, a de-alcoholized one. Sugar is often suggested as substitute, and while it's OK when it's just sweetness you are looking for, it lacks the complexity and nuance of mirin which is required to achieve subtly balanced Japanese sauces, stocks and dips. (There are some nasty ersatz "mirins" out there that are basically corn syrup with added flavours – these should be avoided.)