The zest of a citrus fruit is the colored skin of the rind. Underneath it is the bitter white pith and, then, the fruit. This zest contains aromatic, volatile oils that are used to season food. If you squeeze an orange rind toward a candle you can see these oils light.
Zest can be removed from the fruit with a vegetable peeler or a paring knife. In either case, you might remove some of the bitter white pith along with the peel. If so, just remove it with your parer.
Zesting also can be accomplished with a box grater. To avoid losing much of your zest in the grater, first cover the fine grate with a piece of plastic wrap. Then go ahead and grate.