The umbilical cord serves as the link of life between the fetus in the belly and the mother. The umbilical cord is a tube that connects the two. Through this tube, the fetus receives oxygen and nutrients from the mother and grows inside the belly. The fetus also passes excretions, such as pee, through this tube into the mother's bloodstream.
This tube remains attached to the fetus when it is born, but once it is outside the belly, it is no longer needed to breathe and pee on its own. The umbilical cord is then cut, and the remaining umbilical cord is naturally removed. The umbilical cord is then called the navel. Thus, the navel is a remnant of the connection with the mother.