Could you explain this for someone without any hardware experience?
In the case of electronic CAD, a "part" is what the part looks like in the schematic diagram, typically a box with numbered and labeled connections, and a representation of its physical dimensions and the location of its connections. Somebody has made sure that the physical details are correct, and that pin 23 on the schematic is pin 23 on the physical board.
Creating these parts is kind of a form of costly drudge work for designers, and errors in those parts can be hard to correct when your prototype doesn't work for some mysterious reason.
If you wanted to build a circuit board, you would need a digital model of each component, and these are often made from scratch because designers can't find one readily available. Likely someone at a nearby company, or even a nearby cubicle, has already made that model. So this wasted effort hinders innovation and is a huge productivity drain.
SnapEDA makes a huge database of these models available that hardware designers can drag-and-drop into their designs.