Your belly button marks the spot where your umbilical cord was once attached. This cord is a soft, bendable tube that carried nutrients — vitamins and minerals — from your mother to you, back when you were in her belly (womb). A belly button is also called a navel (Kidshealth).
Obviously not ashamed of what looks like a deformity, the lady showed her elongated navel as she danced happily while giving the middle finger to whoever it may concern.