A quantum particle isn't exactly a solid chunk of stuff. In some ways, it's more accurately pictured as a ripple in the fields that make up the universe.

It so happens that these ripples only come in whole-number sizes (they are "quantized") — so you can have no electron, or one electron, or two, etc., but you can't have half an electron or seven-fifteenths of an electron.

(That's what makes it quantum physics. In classical physics, there was never any rule that you couldn't split a chunk of matter down indefinitely. That turned out to just be wrong.)

What are "fields", though? A field is just some quantity measured at each point in some space and time. An example of a non-quantum field is the wind on the surface of a planet. The wind has some value at every point on the globe. (It might be zero, if the air is still.) And it changes over time. A breeze and a tornado are two different phenomena in the wind field.

(The wind is a vector field; at any point, it has both a magnitude and a direction. Air pressure and temperature are scalar fields; at any point, each has only a single numerical quantity. Gravity is a vector field; mass density is a scalar field. There are other more complicated kinds of field too.)

Why does it matter that particles are ripples and not tiny solid chunks? Well, one reason is that a ripple is inherently spread out, whereas a solid chunk has an exact single location.

And while solid chunks can only interact by bumping into each other, ripples can interact through interference — they can reinforce or cancel each other out. Solid classical-physics chunks can't do that.

(For another non-quantum example: Interference between ripples is also how, for instance, noise-cancelling headphones work: they make a sound wave that cancels out the noise.)

It's this interference that explains a lot of the "weird" effects in quantum physics, like the double-slit experiment. It's not that a particle "really is" a solid chunk that's magically in two places at once; rather, it never was a solid chunk at all, but a ripple that can interfere with other ripples, including its own echoes.