Its different from your waters breaking. Your plug is what seals the cervix, stops infections, stops it from dilating etc. If you lose your plug then it doesn't mean labour is imminent and can even reform. Your waters can still keep intact if your plug has come away.
I didn't lose my plug with both my boys labours as my waters were broke for me with my first and they went on their own with my 2nd. But for all i know bits of my plug may have come out with my waters or i could have gradually been losing over the last couple of weeks.
Correct me if i'm wrong but i *think* this is what happens.