Zest is the peel of citrus fruits used to flavor food, most often made from oranges, lemons, or limes. I use orange zest all the time, such as in my blueberry orange compote and orange cranberry scones. The zest should only be made from organic fruits because it is the outer layer you're peeling off. You can get the zest with a grater or peeler, but is worth investing in an actual zester (as pictured above) if you use zest very often because they are extremely easy to use. Gently grate the outer layer of the peel, being careful not to scrape off any of the white bitter part underneath (the pith). Also be careful to keep your knuckles out of the way (I have made this mistake more than once). If I use an orange and don't have immediate use for the zest, I freeze it for later baking use. Where I live, it is orange season right now, so I've zested dozens of oranges and put the zest in the freezer to last me the rest of the year.