The major theme of Einstein’s 1905 paper, On a Heuristic Point of View Concerning the Production and Transformation of Light, was that light (under certain circumstances) behaves as if it’s comprised of individual particles rather than waves. These particles, or “chunks” of light were originally called light quanta, and then later came to be called photons.
It was the first of Einstein’s 1905 papers, On a Heuristic Point of View Concerning the Production and Transformation of Light, which he referred to as “very revolutionary” – the only time he would ever say this about any of his work, in fact – and which, in part would win him the Nobel Prize in 1921.
At the atomic level, energy occurs as “chunks” or quanta.
Transitions between the energy states of atoms and molecules require a specific amount of energy, nothing more, nothing less; it’s only these particular values that are allowed by nature.
The energy states available to atoms and molecules occur at specific intervals. In other words, they are discrete rather than continuous.