The mucous plug. The famed clump of cervical mucous that is fabled to herald imminent labor and delivery.
The first sign of labor, according to every woman online, in your playgroup, the line at the grocery store, and in your church congregation. If you "lose your plug," it means labor and delivery are imminent, and you must call and or go to the hospital NOW! 
The mucous plug is just what it sounds like: a plug of thickened, often hardened mucous in the os or opening of your cervix. It begins to build up shortly after the first trimester. Here's the secret very few people know: you can lose it as early as 28 weeks, and it doesn't mean diddly in regards to labor.
Now, I know that every message board known to man will have on it that one of the first signs of labor is the loss of the famed mucous plug. And you all have friends,neighbors, and relatives who will tell you they lost their plug and bada-bing, went into labor merely hours or minutes later. Well, that's great. Good for them. 
It still is not a sign of impending birth. Or labor. Or anything exciting. You can shed your plug for no reason at all, and it is not harmful to you or the baby if you lose it early, as it often regenerates.
Loss of the mucous plug merely indicates some sort of ripening or irritation of the cervix. Sex. A yeast infection. Bacterial vaginosis. Lots of braxton hicks contractions. Or labor. It could be ANYTHING. 
You can lose your plug and still remain pregnant for days, weeks, months to come. If you lose your plug, it is not a sign that labor is imminent, or even relatively close. It's like saying that contractions mean you're in labor, which is of course, false. 
Spread the word, ladies.Pass it on to your friends and neighbors. 