It is what keeps things in, liquids etc when you are pregnant. When you are going into labour you lose your mucus plug usually right before labour. It will start with a little bit of pinkish blood or mucus. It might happen early or right before your water breaks. It's exactly what it sounds like it's a plug of mucus..
It blocks the opening of the cervix to prevent bacteria from entering the uterus. Before labor, It comes out, allowing the baby to pass through the cervix during labor and birth.
Its this muces stuff that is around ur cervix its like a plug to keep it nice and tight shut not sure how to better explain it