Candling is a technique used to examine the growth and development of the embryo inside the egg. A bright light is placed behind (or underneath) the egg in order to see details inside. Although candles are not generally used anymore, the term still remains.
During the 21 day incubation period, I would candle the eggs once or twice in order to discard eggs that weren’t fertilized or developing. Although commercial candling set-ups are available for purchase, I asked around the farm and found there are simple and very inexpensive homemade candling options available. My favorite candling set-up was a cardboard box, LED flashlight and roll of toilet paper. When I was ready to candle, I would first set up my lab. This involved shutting the blinds and turning off the lights in the Polyface Farm store office, borrowing the brightest flashlight I could find, getting the box from the box discard pile, obtaining a roll of TP, and getting the eggs out of the incubator.
After about a week in the incubator, you should be able to see veins developing. After a couple weeks, you can see the embryo forming.
There is a lot of information available on the internet and in books about candling. Brinsea, the maker of the incubator we use, has a more comprehensive article on candling here.