"Candling" is when the egg - in its shell - is held against a strong light, enabling you to see any interior imperfections such as blood spots or - if you happen to have a rooster around - a developing embryo.
Since commercial egg producers don't keep roosters, they candle eggs to pick out & discard any that have blood spots (which, by the way, have absolutely nothing to do with the egg being fertile or not). That's why you rarely, if ever, come across blood spots in commercially-produced eggs.
I never bothered candling my hens' eggs because 1) even tho I did have roosters, we collected eggs several times a day so there was no chance of any of them turning into chicks, 2) blood spots, while perhaps not appealing to look at, are perfectly harmless, & 3) if we did happen to come across the occasional egg that had been laid in an out-of-the-way place & might have been there for awhile, it was easier just to discard it.