Explain 3d printing to me

I’ve been reading on this and not really had the time to even look into it. Heck it took me three months just to get powder, casings and bullets to reload again. Wth is 3d printing? I see today a lower court has thrown out the ATF definition of ghost guns. Good now it goes to the fifth circuit and to heck with them now they have to defend their overreach. Thanks in advance…. I can’t seem to grasp hey I am going to print a plastic gun and be all good with chamber pressures and ballistics and all?