Question

What does it mean that energy is quantized?

Answer 1

Quantization is the concept that a physical quantity can have only certain discrete values. ... For example, matter is quantized because it is composed of individual particles that cannot be subdivided; it is not possible to have half an electron. Also, the energy levels of electrons in atoms are quantized...
Rather than continuous waves, light was coming in packets of energy which can be defined with its frequency and planck constant 'h' as we mentioned earlier and said 'so, the energy is quantized'. In the atomic world, energies are also quantized. This means it can have only certain values.