It’s a lot more complicated than that, but essentially works out to the same thing. Certain subatomic particles have bizarre properties that defy our standard rational understanding of the physical world. They don’t actually exist in one particular place at one particular time, unless they are observed. So, an unobserved electron traveling from point A to point B can’t be visualized as a ball flying from one place to the next. Instead it’s more like a wave, with the peaks and troughs representing possibilities as to where the electron may travel. Technically, it’s actually traveling through all possibilities.
?????? ??? ???. Because it gets weirder.
Now, using classical Newtonian physics, we “know” that the particle must be somewhere along that wave. Like in the double slit experiment, it either travels through slit 1 or slit 2. Right? So we put sensors on each slit to see which one it goes through. Alright, now the sensors are sending back data telling us which side the electron went through. Except now that the electron-wave isn’t a wave anymore, it’s a particle, acting like a ball being thrown through a hoop. How does it “know” when it’s being observed? It has nothing to do with whether someone’s eyeballs are on it, or if someone watched the DVD, or any of that metaphysical nonesense. It’s merely a question of whether something “hit” the wave on. The sensors give off photons (or other equivalent sub-atomic particles) which interact with the wave, forcing it to stop being a wave and start being a particle.
Imagine, if you will, watching waves crash ashore on a beach. Now imagine that if you stick your hand in a wave, the water disappears and a beach ball appears, sometimes in your hand, sometimes a few feet away. That’s sort of what’s happening.
Confused yet? Good. It means you’re starting to get it.