In quantum mechanics observation changes the thing observed, but that doesn't mean you can use that to cast spells that do whatever folk claim to do with magic. Here's how it works:
Imagine you're in a dark room. There are objects in that room. You have a big pile of baseballs and want to know where the objects are in the room. You can throw the baseballs in various directions. In certain directions the baseballs bounce back. When they do, you know they hit some object. You can then use things like the speed and direction in which you threw the ball and how long it took to bounce back to tell you in what direction and how far the object is. Throw enough balls and you can get some idea of the size and shape of the objects.
Now, all the objects in the room are on casters. When you hit them with a thrown ball, they move. If they're big and heavy they don't move much. If they're small and light they move a lot. Observing them, by throwing these balls at them, is going to affect them.
Consider the balls themselves. The size of the balls limits what you can "see" with them. Anything smaller than the ball itself you might see that it's there, but you won't be able to tell it's size and shape. And when you hit it with that ball it's more likely to go sailing across the room and you only know where it was, not where it is now (after it got hit). If you want to see smaller objects, you need smaller balls. Now, this is the tricky part. There's a rule. The smaller the "ball" the heavier it has to be. That's backwards from what we usually think of things, but to describe quantum effects you need that rule.
So you can see smaller objects by using smaller "balls", but the result is that you're going to hit the objects harder with those heavier balls and knock them just that much farther and faster away.
This, right here, is "observation changes the thing observed." The balls are whatever we use to look at something, whether sound waves, quanta (discrete packets) of light, electrons in an electron microscope, or anything else. We "shine" the light or whatever on the object we wish to see (throw balls at it) and look at either what's reflected or what passes through it to "see" the object.
At a basic level, when it comes to light the size of the "balls" (the wavelength of the light) is given by the following formula:
Eλ = hc
E = energy
λ = the wavelength (size of the "balls")
h = Planck's Constant a really, really, really small number. (Okay, it's
6.62606957 × 10−34 joule∙second, but at this level what you need to know is that it's really small.)
c = the speed of light.
For "particles" like electrons that have mass, the equation is a bit different:
λp = h
Here p = momentum.
In both cases, to get a small wavelength (small "balls" to look at small stuff) you need to have either a high energy (light) or high momentum (particles with mass). Heavier balls that you throw harder. And, when you throw heavier balls harder at the thing you're observing, you knock it around more.
That's "observation affects the thing observed." It's not magic. It's the simple fact that to observe something you essentially throw balls at it. They're just really, really tiny balls (see that Planck's Constant). And the effect is only important on really, really small things, things like electrons, sometimes atoms themselves. To affect larger things that way, you need a bunchaton of energy.
This analogy only scratches the surface. There's a lot more I could do. (Quantum tunneling: the balls are "squishy" and can sometimes get through holes that are nominally too small for them.) But that will be enough for now, I think.
And if you liked this, you may like my novel Survival Test: