Mucus plug?

So I never really paid attention to all the mucus plus talk because I never even saw it with my first. But after having on off contractions for two weeks now, back/hip/pelvic pain and just being all around miserable, and to top it off I expected to have given birth already considering my first came at 38 week and I’m now 38+1. I lost my mucus plug this morning, like a giant chunk that kept coming. My questions are, first what does this mean? Labor? Nothing? Also am I supposed to inform my doctor? Or is that only in cases of preterm labor.