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. (For more on how 3D printing works, see our article at T. Rowe Price Connections.)