What does the word Quantum mean? The term quantum was coined by Andre-Marie Ampere (1822,) and was later adopted by Albert Einstein, to describe the wavelike-particle nature of light. He employed a variation of the word "quanta" (now called photons) to describe these wave-particles of light. Lewis De Bruglie (1924,) later described the movement of subatomic particles similarly, characterizing them as also being wavelike in nature. Werner Heisenberg added his "uncertainty principle" and the world of modern quantum mechanics was beginning to take shape.
Quantum mechanics (defined by Wikipedia) is "the study of mechanical systems whose dimensions are close to the atomic scale, molecules, atoms, electrons and other subatomic particles." Research in this field is now heavily concentrated on more minute "quanta" (now used as the plural of quantum) is known to be the smallest indivisible particle, the equivalent quantity with the same units as the Planck constant: related to fermions, bosons and photons.
Quantum theory is most valuable, because it accurately describes and predicts the mechanics and dynamics of minute scale particles. It actually works! Unlike other theories, Newtonian physics breaks down (hopelessly) at the particle level. It has been theorized that if Newtonian physics were active in quantum scale, the universe as we know it could collapse into a jumbled mess of super compact atoms and electrons. Matter would cease to exist in an orderly format.