In order to wrap your head around this, file away what your mind tells you about printing or pictures. A normal printer squirts ink onto paper, but a 3D Printer layers atoms on top of each other to create (or print) actual objects.
I can explain this in more detail by taking us through the history of the 3D-Printing industry. What we now call 3D Printing was called rapid prototyping for many years. An engineer would design an object as a CAD (computerized aided design) file, and then send that file to a machine to produce the real thing. (See our accompanying article, How 3D Printing Works.)
But for years, the plastics and the metals that were used were just not robust enough to create a prototype that you could be proud of. They resembled paraffin waxes. They could create the parts, but those parts tended to be flimsy. Because the end product didn’t have structural integrity, the technology was really just for engineers who were creating a product in CAD and needed to see what it looked like in real life.
The revolution took place when companies like 3D Systems started designing radically new materials. (See the article Substance Before Form for more.)