This is a very good article that explains the different types of umbilical hernias.
BreederVet - Umbilical Hernias in Dogs
If the umbilical ring does not close immediately after birth sometimes a little fatty tissue is trapped on the outside of the abdomen after the ring closes causing a bubble-like area where the umbilical cord was attached. If the bubble cannot be pushed back up into the abdomen it is called a non-reducible hernia. If the bubble can be pushed back up into the abdomen then it is called a reducible hernia and surgery is advisable.
I had a female once that I though had a hernia that was about the size of a marble. The vet operated and it turned out not to be a hernia, just some fatty tissue trapped.