Learn more about what evaporated milk is, and what to substitute for it when cooking. 
Evaporated milk is milk that has been cooked down to allow some of the water content to evaporate. The concentrate is canned, and the result is a heavier tasting milk with a slightly toasted or caramelized flavor. If you don't have any, make your own: To produce 1 cup of evaporated milk, simmer 2 1/4 cups of regular milk down until it becomes 1 cup.
In many recipes, evaporated milk may also be replaced with a combination of whole milk and half-and-half. For 1 cup of evaporated milk, use 3/4 cup whole milk and 1/4 cup half-and-half.