Quantum theory was first presented in 1900 by the scientist Max Planck. He represented Energy as individual units called quanta. This was the beginning of Quantum Theory and Quantum Mechanics (Physics).
Quantum Physics explains the nature and behavior of matter and energy on the atomic as well as the subatomic level. The state (list/make up of properties) of Quantum matter is not determined until an observation is made. Quantum objects are entangled, meaning that the properties of one object cannot be separated from the other. Therefore, the observation of one Quantum object depends on the observation of a different Quantum object somewhere else. Basically, the rules of Quantum Physics state that an unobserved photon (or other Quantum object) exists in all possible states simultaneously, but, when observed or measured, exhibits only one state.
This is just my basic understanding of Quantum Theory and Mechanics. I don't claim to know a lot about it. Most of it is above my head.