"Zesting" / "to zest" means to take off the zest, the thin outer layer of peel on citrus fruit (lemon, orange, grapefruit, etc.), to add the fruit's flavour with a hint of bitterness to a mixture.
This can be done with an ordinary knife, a vegetable peeler
, or a more specialised utensil: a zesting knife or grater.
For more details, see the page on how to zest a fruit
"Zest the lime into the cream, then mix well."
Meaning: Use whichever utensil you prefer to remove all the zest from the lime and add this to the cream, then mix in well.