A particle's location in quantum mechanics isn't precisely defined, it's more of a bell curve. It's most likely to be in the center, but there's a non-zero probability that it could be anywhere in the universe.

This means that if you put an "impassable" barrier somewhere close enough to the center of a particle's bell curve, you end up with a significant chance that the particle could be on either side of the barrier.

This behavior is confined to the quantum scale because in order for it to happen to a macroscopic object, all of its gajillions of particles would have to tunnel at exactly the same time, making the probability functionally zero. Furthermore, any macroscopic barrier would also be far too thick to be within the distance tunneling can realistically be expected.