At one time losing your mucus plug (also called "bloody show") was considered a sign of imminent labor, but doctors have discovered it's not that reliable at all. You could go hours, days, even weeks before delivery day. Some women don't even notice when they lose their mucus plug.
As the name implies, a mucus plug is a slightly gooey, sometimes stringy, clear glop (sorry I can't come up with a better description). The plug may be streaked with blood or slightly pink—that's normal.
The American Pregnancy Association (APA) points out the very important function of the mucus plug, which is to "[block] the opening of the cervix to prevent bacteria from entering the uterus" during pregnancy. As you near delivery, the cervix will begin to dilate and thin so that your baby can make her way through the birth canal. Losing the mucus plug is part of that process of getting ready for delivery.
If you discover clear fluid, instead of sticky mucus, you may be seeing amniotic fluid instead of the mucus plug. You should contact your health care provider right away if you suspect you're leaking amniotic fluid. Otherwise, you can probably wait until you're regularly scheduled prenatal visit to let your provider know about losing your plug.