It is possible to buy fertile eggs at the store, but unless they specifically say they’re fertile, they’re almost certainly not.
Fertile means that the hen simply had access to a rooster who did his job – i.e., in a barnyard or something. (Most “regular” eggs come from factory-like enclosures with six hens to a tiny cage, whose feet are formed to the bars of the cage and have no room to turn around and a cauterized beak so they don’t peck and kill the other hens, so it’s unlikely that a rooster is going to get in there to give them some sweet lovin’.) It does not mean that you will see a little fluffy baby chick when you crack it open. (At most, it would have a small ‘bloody’ looking spot attached to the yolk.)
A lot of people buy them because it implies a happier life for the hen, who probably lived in a barnyard with a rooster and other hens, rather than in a factory.
They are more likely to be found at places like Whole Foods or farmers’ markets.
Okay, done editing. :)