In cooking, zesting one lemon means to take the outermost peel off the fruit. This can be accomplished by using a tool called a zester.
Zesting can be done to different types of citrus fruits, such as citrons, limes, oranges and lemons. The outermost skin of these fruits is filled with strong citrus flavor, which can be added to many different dishes or used as a garnish.
When zesting, it is important to only remove the outermost skin from the fruit. The white part directly under the colorful skin is called the pith and should be avoided because it is very bitter.