It's related to the word "plastic", which refers to materials that can be molded or shaped.
Here are some examples of words with the root "plas":
* Plasma: The fourth state of matter, which is a highly ionized gas. It's named for its "moldable" nature.
* Plasmodium: A genus of parasitic protozoans that cause malaria. The name refers to its ability to change its form.
* Plastics: Synthetic materials that can be molded into various shapes.
* Plasticsurgery: A surgical specialty that deals with the repair and reconstruction of body defects.
Therefore, "plas" essentially refers to the concept of formation or shaping.