If the egg has been fertilized, then it has everything it needs to become a baby bird. But it must be kept warm and turned over occasionally. That's why the mother normally sits on the eggs. If it's kept at the proper temperature, then eventually the baby inside will peck a hole in the shell and wriggle out. If that happens, you will have a marvelous sight to see, but it will be the beginning of a whole new set of problems: You don't have anything that a baby robin can eat, you have no way to feed it, and you can't keep it warm and clean. Sadly, even if you get far enough to see it hatch, it probably won't survive.
