The 3D printer, heats a relatively low-temperature plastic filament to build parts layer by layer using designs supplied to the machine. The goal of having the printer on the orbiting laboratory is to demonstrate that additive manufacturing can be used to make a variety of parts and tools in space, reducing the need to send replacements from Earth.
Typically, it can take months, or even years, to get something up to the ISS, depending on how the resupply missions are scheduled. However, additive manufacturing technology could get tools and parts in the hands of astronauts in a matter of hours. This could be especially helpful in an emergency situation. When developing the printer for space travel, the first consideration was how it would be affected by gravity.
However, the team found that, without the pull generated by gravity, they were able to produce certain prints more easily in space. Secondly, the printer had to be designed for the enclosed environment of the ISS. When something is shipped to space, it has to be reviewed and examined before making the journey. With 3D printing, though, NASA will have to approve the items without actually seeing them, since they will be printed on board the ISS. NASA does currently approve and send the plans for what can be printed on the 3D printer. The samples produced by the printer will be sent back to earth for further examination and study.
NASA has released its own open source files for 3D printing that you can download at https://nasa3d.arc.nasa.gov/.