Learn how to make a homemade egg candler using common household materials. Backyard Chickens has a tutorial for how to make a diy egg candler. Candling eggs is done to determine if the chick is developing inside the egg.
This was done back in the day using a candle to provide the light to see in the egg. Now it is done with either a regular light bulb or LED lights. This way you can check to see if your eggs were fertilized and are developing or if they are not so you can stop wasting time on that or those eggs and get some that are fertilized. It was an interesting article because they have a bunch of picture of egg being candled and you can see the little chick growing in the egg.