What does it mean when my belly button pops out?

Navel: The navel is a scar where the umbilical cord was once attached. Around 90% of people have a depression (innie) ; about 10% have a protrusion (outie). It is reported that a protruding navel develops because of more abundant skin remaining after the umbilical cord is separated. A naval that pops out may be consistent with an umbilical hernia which would warrant medical eval.

About 10% of: Individuals have a protruding navel (outie). However, what you described could be an umbilical hernia. Your doctor should be able to diagnose if you have an umbilical hernia.