Zest is the name for the rinds of citrus fruits that is used to add a lot of flavor to lots of things like cakes and breads. Only use the outer colorful part because the white part inside is bitter and nasty. What you do is to grate the outer part using a hand grater or a food processor and whatever you're cooking will taste like that food. You can buy zest in jars at the supermarket but by that time the zest will have lost its flavor. However you can keep zest in a Ziploc bag and put it in the freezer and it will last a long time.