3-D printers are a widely used machine. Originally, the printers were used to make things out of plastic, but now 3-D printers can make lots of things in many materials. 
3-D printers start out with a CAD(computer aided design). The design is then sent to the printer where the printer starts to melt plastic. The melted plastic then goes through a small tube that starts to create the 3-D design.
This is related to engineering because 3-D printing is used for many things that are related to engineering.
