Candling an egg is a way to look at the contents of an egg without breaking it. When a company sell free range eggs and organic eggs they need to know that their eggs are top class. Also if a chicken farmer is growing chicks from eggs he may want to check on the progress and health of the peep (baby chicken still inside the egg).
Candling is very easy – you shine a very bright light onto the egg – from close up – and then look from the other side. White eggs are easy – you can see if there is a baby inside, or if there are blood spots in the yolk. You can also tell how big the air sack is – the bigger the air sack the older the egg. What also shows up is the shell porosity – all of this from shining a very bright light and looking from the other side. All free range eggs, in fact all eggs should have a sample of the batch candled.