Most times losing your mucus plug means that you're getting close to going into labor and are almost ready to give birth to the baby.
The mucus plug is there to protect the baby from the outside elements while the baby is inside you. It's a protective plug for the baby.
It is however possible to lose your mucus plug and still not go into labor so it doesn't always mean you're ready to go into labor with the baby but most times it does mean you're about to give birth soon to the baby.
When you lose your mucus plug you should go see your doctor so they can see if you're about ready to give birth or not and make sure everything is okay with you and the baby.