How does 3D Printing work?
The most common 3D Printing technology is called Fused Deposition Modeling (FDM) which lays down very thin layers of plastic material called filament, one by one in it melted state. The material can then be extruded on a specific and predetermined path created by the software on the computer that instantly cools down and solidifies providing the foundation for the next layer of material until the entire object is created. Therefore it is a very slow process when a high print quality is desired.
Here's a video link of how 3D Printing (FDM) works: