What is quantum theroy with easy exmple

Quantum theory is the theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level. Planck wrote a mathematical equation involving a figure to represent these individual units of energy, which he called quanta.

Example:

Quantum mechanics [QM] is a branch of physics which describes physical systems so that properties like the energy or angular momentum are discrete quantities that are multiples of a smallest unit or quantum. A famous physicist named Erwin Schrödinger made an example of quantum mechanics, often called Schrödinger's cat.