Zest is a technical term for the thin peel of citrus fruits such as oranges, lemons and limes, without even a hint of white on it.
If you need zester for a recipe, peel the fruit as thinly as possible. You can do this with a sharp potato knife, but even better with a peeler. Large chunks of the skin should then be cut into strips or pieces. Or run the fruit over the coarse grater (and do not press too hard). Result: grated peel or, to be more precise, grated zest.
Of course, for preparations in which the zest must be included, use untreated fruit that you first scrub thoroughly with hot water.