The rind of a lemon, or just about any other citrus fruit, is typically made up of two layers. The pith is white and bitter, and it is sandwiched between the zest and the flesh of the fruit. Zest of a lemon is the bright yellow part of the rind. It is the thin outer layer of the rind, and it is packed with fragrant and flavorful oils and juices.
Before using a lemon zester on any type of fruit, culinary experts agree that it is important to wash the fruit and dry it thoroughly. The metal grating part of the lemon zester is then pressed on to the top of the fruit and dragged down the rind, often from top to bottom. Some more experienced zesters are able to make long continuous threadlike peels of zest, which are great for garnishes, by running this tool in a downward spiral around the fruit.