Hi: Lemon zest is basically the outside of a lemon that has been grated off (you can get a tool for zesting or even use a cheese grater). Some tips I found online:

• Lemon zest is the most aromatic and flavorful when first removed, so use immediately.
• Always remove the zest first before halving and squeezing the lemon.
• Thick, bumpy textured cold lemons give the maximum amount of zest.
• Lemon zest, depending on its use, comes in many forms: long wide strips, julienne, or finely grated. Use a potato peeler for strips, a zester for julienne, and a grater for finely grated zest.
• When removing the outer rind (zest) do not remove the white pith, which is very bitter and inedible.