EXPIRATION DATE A date on an egg carton beyond which the eggs should not be sold. see Carton Dates
The "sell by" or "best if used by" or "exp (date)" are all expressions used by the industry in various states, and are used by the retailer to assure you of freshness. The egg will continue to be fresh for at least another 2-3 weeks if it has been refrigerated from the time packed until used at 45 degrees F. or lower. As the egg ages, it does lose some of its qualities, so if you were baking a cake or whipping meringue, your cake might not rise as high as expected, and you might not get the volume of meringue you would expect, so for baking purposes it is better to use a fresher egg.
Older eggs are great to use when hard-cooking (boiled) as they will peel easier. They will keep for about a week in the shell, 2-3 weeks if peeled and placed in a freezer bag.
When frying an older egg, it will spread out further in the pan, as the white (albumen) gets thinner. When fresh, the yolk will stand high and the white will stay close to the yolk in your pan. Nothing wrong with cooking the older egg, it just doesn't look as nice.
The term "rotten egg" seldom occurs today, thanks to refrigeration. The interior of the egg will probably evaporate before it becomes "rotten". We have seen eggs several months old still very usable, but could not recommend you that you do this!