Wave-particle duality
Every quantum object is both a particle, like a tennis ball, and some sort of wave, like a wave in the ocean.
Imagine a tennis ball that would be in several different places at the same time. But when you try to locate it with measuring instruments, the quantum object is suddenly reduced to one spot. This means that electrons, atoms, molecules, and even photons (light particles), are small particles and waves, both at the same time!
This is the basic property of the quantum world.