Mirin is sweet rice wine widely used in Japanese cooking. It is mainly used for cooking, but some are good enough to drink. Mirin is made from steamed Mochi rice and malted rice that are fermented with added alcohol. It has a higher percentage of alcohol in it than you might think (about 14%). In the process of fermentation, rice produces a lot of sugar along with alcohol and turns out very sweet. Mirin gives food sweetness, shine, and a good aroma. It is used like cooking sake, but it has a much sweeter taste which is perfect for Chicken Teriyaki, stewed meat and vegitables (Nimono), Mentsuyu sauce, and various other dishes in Japan.
If you have limited alcohol intake, you can substitute with sugar and water (1:3), or a smaller amount of sweetener such as honey or agave syrup.
You can buy Mirin at a lot of US supermarkets in the Asian food aisle or online.