Mirin is a type of Japanese sweet rice wine. While the description sounds a lot like sake, mirin is much sweeter and contains far less alcohol. When used in cooking, mirin can provide the hard to describe sweetness and tang that’s typical in Japanese cooking.
If you’re shopping at a major grocery chain, you’ll most likely find Kikkoman brand “Aji-Mirin.” Unfortunately, Aji-Mirin (pictured on the left) tends to use added sweeteners like corn syrup compared to more traditional or “authentic” mirin that uses cane sugar.
If you have other options, I would suggest something more high-end or at least one without corn syrup. The picture above was taken at my local Whole Foods. The regular mirin is on the right. While slightly more expensive, I would recommend springing for real mirin. If you’re not sure you want to invest in something more expensive, or if it’s the only option available, Aji-Mirin is a good starting point.