It just means something that was not deliberately planted. Usually from a seed left in the ground , dropped by a bird or other animal or a sprig of plant inadvertantly dropped. Example if you sit on your back porch and spit watermelon seeds into your flower bed, you will most likely get lots of volunteer watermelon plants.
I have volunteer tomatoes growing. I did not plant them. Last year volunteer tomato plants grew in the same spot. Some of their fruit was not picked and fell to the ground. The seeds from those tomatoes survived the winter and have sprouted. Some of these plants now have tomatoes.
These are known as "volunteers".
I don't think "clippings" would be considered volunteers as they are deliberately taken from a mother plant and set into the ground (or pot) to reproduce. These (I'm guessing) would be considered "clones".