What does it mean that energy is quantised?

When we say that energy is quantised, it means that energy exists in discrete packets or "quanta" rather than being continuous. In quantum mechanics, this concept implies that systems can only possess specific, fixed amounts of energy, often associated with particular states or levels. For example, electrons in an atom can only occupy certain energy levels and can transition between them by absorbing or emitting energy in quantised amounts, such as photons. This quantisation underlies many phenomena in physics, including atomic structure and the behavior of subatomic particles.