A cookie (or cookaloris to give it it’s full name) is something placed between a light source and the surface that light is intended to fall on. It’s purpose is to make the light more interesting by introducing shapes for the light to pass through. The above pic is a demonstration using a glass of lemonade. Usually the lemonade would not be included in the picture.
The above is intended to eliminate any snarky remarks about little things and little minds.
Note also that this lemonade is Shweppes Lemonade. It probably wouldn’t be as good with a cheaper brand.