when you where in the womb, your mother provided nutrients to you via your umbilical cord, which connected to your stomach. when you where born, the cord was snipped, and the remaining fell off when you were around two or three weeks old, forming a divet at your stomach called your belly button. so sense it was the first way you ever got nutrients (aka food) into your body, it is like your first mouth.
sorry that explanation was so long. i hope it makes sense now!
♪~ ᕕ(ᐛ)ᕗ