The mucus plug is found in the uterus and is intended to block bacteria from getting into the uterus.
As the delivery approaches and the cervix starts to open, you may find bits of it in your panties. The release of the mucous plug is not necessarily a sign the delivery has started, only that it is approaching.
You have probably already written a birth plan letter? If not, this is now becoming urgent. Write down the thoughts you're having about the delivery in the letter. For example, what pain relief are you willing to take? Here you should write down things you want to avoid and things you would like to try. It's hard to know exactly how you want things to be. Sometimes, things you had strong feelings about before delivery do not feel as important once you are in the delivery theatre. Think of the birth plan as mental preparation, allowing you to think about the delivery and how you'd like it to be. The birth plan letter becomes a compilation of what you think is best for you the day your child will be born - which may happen anytime now?