If you look down at your stomach, you can see your navel. It’s that weird-looking round depression in the middle of your belly, unless you have an "outie," in which case it pokes out instead of in.
Your navel is actually a large scar that was caused by the removal of the umbilical cord when you were born, also known as your belly button. The word can also be used figuratively for a center point, like how the navel is in the center of your stomach. You might call New York City the navel of the arts scene in the United States. Careful: navels have nothing to do with the Navy, that would be naval.