An extremely specialized area of physics that is really hard to understand, but it does actually make sense when you study it. Trust me, it's fucked up and involves a lot of fucked up math which I'm not going to explain but some of the other definitions on here explain it.
It will fuck up
your worldview big time and that's one of the reasons Albert Einstein hated it so much. Can't blame him, I mean, his
theory of relativity was supposed to be the one which finally described the true nature of reality and then comes along the double slit experiment and the
schrodinger equation which just completely
butt raped our current understanding of reality at the time and it's still puzzling today, although many scientists are searching for a grand unified theory which should explain the relationship between relativity and quantum mechanics.
Some of its philosophical implications are fucked up. Many believe it means reality doesn't exist but only our minds do, some believe it means there are parallel timelines/universes to our own, some believe its due to higher dimensions geometrically interacting with ours, some even believe it could imply our universe has some sort of pan-psyche to it. Yeah it gets really fucked. It just goes to show that we don't know shit about our universe.