To zest a lemon means to remove the outer layer of the lemon peel, which contains the aromatic oils and adds flavor to dishes or drinks. Full definition
bad boys in your kitchen drawer, you can shred green papaya for a Thai salad, peel potatoes for mashing, or even zest a lemon for a drink. (bonappetit.com)
Zest the lemon with an official zester, or peel just the thin yellow layer with a peeler and then chop very finely — remember to avoid including the bitter white membrane. (mixedgreensblog.com)