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You can think of a plug as almost the opposite of a mold. With a mold, the fiberglass is laid-up on the INSIDE surface. This means that when the part is pulled out of the mold, the smooth or "finished" side of the part is its OUTSIDE surface. Sometimes you need the INSIDE of the completed part to be "finished" instead. For this you use a plug and lay the fiberglass up on the outside of the mold so when removed the INSIDE of the part is smooth and finished.
For example, if you are making a fiberglass bust of a horse, you would use a mold so that the visible side is "finished" when removed. But if you are making a large food grade container for a bakery, it is more important that the INSIDE of the container is smooth and finished. In this case you would use a plug to lay the fiberglass up on.
