Quantum tunnelling is the effect in which a particle passes through a barrier that classical physics says it lacks enough energy to cross.
In quantum mechanics, a particle is described by a wavefunction rather than by a single classical path. When the wavefunction encounters a barrier, it generally decreases inside the barrier but does not immediately become zero. If the barrier is sufficiently thin, a nonzero part of the wavefunction appears on the far side, giving a measurable transmission probability.
Tunnelling is central to alpha decay, where an alpha particle escapes an atomic nucleus. It also powers the scanning tunnelling microscope, enables current through tunnel junctions, and helps drive nuclear fusion in stars. The probability falls rapidly as the barrier becomes thicker or higher.
Tunnelling does not mean a particle gains extra classical energy or travels through a hole in space. It is a direct consequence of quantum wave behavior.