Welcome to the group. Nonfat instant powdered milk is just skim milk that has had the water evaporated from it. It is very economical for baking and cooking and much less expensive than fresh milk.
It’s also good as an emergency supply as it will last a long time in the pantry.
You can reconstitute it by adding water and following the package instructions and use it for drinking. Some people don’t care for the “cooked” taste of it but I know a couple of women who combine it with a carton of fresh milk to stretch the amount of fresh milk.
You can also use it reconstituted in recipes that call for liquid milk such as baking, sauces, soups, etc.
There are also recipes that use it an a dry ingredient.