Prior art basically says that someone has thought of it before. What complicates this, is that prior art could be that you released the info yourself before claiming a patent, or it could be something as simple as someone’s brochure for vaporware that was never implemented.
An example. I could put up a website saying that I was going to create a machine with self leveling legs. Now, if a week after I do that Stern tries to patent that idea, it could be rejected due to my prior art. Even if I never produce the machine.