Superwash wool is a special wool product that has been treated or processed in a way that allows it to be machine washable. Many people are afraid to work with wool because it is so easy to shrink (though some of us shrink wool on purpose) and superwash wool can allow them to work with great fibers without worry.
Superwash wool is a wonderful choice if you like natural fibers but are giving something as a gift and don't want to burden the recipient with a garment they have to handwash.
So how is superwash wool made? It can be made using an acid bath that removes the "scales" from the fiber, or it can be made by coating the fiber with a polymer that basically keeps the scales from being able to join together and cause shrinkage.
You probably won't know which method was used on your superwash wool when you buy it in the store, but you might be able to tell when you start knitting. That's because polymer-coated yarn tends to be slicker than regular wool.
To care for superwash wool, wash it on the gentle cycle in your washing machine. A natural-fiber shampoo like you might use for hand-washing non-superwash wools is recommended. Let the garment air dry rather than using the dryer.