Most people in North America first encounter miso as a flavoring for soup broth in Japanese restaurants, a starter before the sushi. Miso hails from Japan and, like soy sauce, is a fermented food that adds salty depth of flavor to savory dishes. There are a few versions, all made from soybeans, possibly with rice or barley. The mildest and sweetest, shiro miso, is white to pale yellow in color and is made from rice and soybeans. There are many misos in the middle range, mild to strong, all medium-brown. In Japan this category is known as "red miso" because when used as a marinade it gives an attractive reddish-brown tint to the food. Finally, the darkest miso, brown in color, with a strong, smoky flavor, is known as hatcho miso.
I like to use mild or medium (white or "red") miso to flavor vegetables before grilling them, a good summer use. But miso is also wonderful if I have a soup, a chicken broth, say, that lacks oomph. It gives a satisfying depth, the flavor that we know by the Japanese word "umami." (Umami is often described as a meaty flavor, like that of cooked mushrooms or ripe tomatoes, meat broth or grilled meat.)
Start experimenting by stirring a tablespoon of miso into a little warm water and adding it to a soup or stew as it's cooking. Or add the miso-water mixture to a vegetable stir-fry a minute or two before it's done. Taste after you add the dissolved miso. If you want to add a little more, do so, again dissolving it in water before stirring it in. You'll notice that it gives more substantial flavor; it also adds saltiness, so season the soup or stew or stir-fry only after you've added the miso.