Quantum tunneling is a phenomenon in quantum mechanics where a particle can pass through a barrier that it classically shouldn't be able to cross. This occurs because particles, like electrons, do not have definite positions and can exist in multiple states at once. When they encounter a barrier, there is a probability that they can "tunnel" through it, even if they don't have enough energy to overcome it. This effect is crucial in various processes, such as nuclear fusion in stars and the functioning of devices like tunnel diodes. Quantum tunneling challenges our classical understanding of physics, illustrating the strange and counterintuitive nature of the quantum world.