Mucus plug is a thick layer of mucus plugging up the cervix so nothing gets to your baby
If your cervix was checked for dilation, that could be the reason you lost your plug btw. Sometimes that dislodges it.
Unfortunately it doesn't necessarily mean anything. You could go into labor tomorrow, or you could going into labor in three weeks. It's the annoying thing about labor, there aren't necessarily clear cues besides contractions and water breaking.