Short answers, pulled from the story.
Max Planck postulated the constant in 1900 as a proportionality constant needed to explain the observed spectral distribution of black-body radiation. He calculated an early value of 6.55 from experimental data, within 1.2% of the currently defined value. Planck received the 1918 Nobel Prize in Physics for his discovery of energy quanta.
Planck later referred to the constant as the "quantum of action". He initially denoted it with a symbol thought to stand for Hilfsgröße, the German word for "auxiliary quantity".
The Planck constant is the proportionality constant between the frequency of incident light and the kinetic energy of photoelectrons. Einstein's 1905 explanation of the photoelectric effect proposed that light arrives in discrete energy packets equal in size to the Planck constant multiplied by the light's frequency, a prediction later confirmed by Robert Andrews Millikan's experiments.
Albert Einstein received the 1921 Nobel Prize in Physics for his work on the photoelectric effect, not for relativity. His 1905 paper showed that the Planck constant governs the energy of light quanta, and the Nobel committee chose that work partly due to bias against purely theoretical physics and internal dissent about the proof of relativity.
The kilogram is defined by fixing the numerical value of the Planck constant at exactly 6.62607015 J·s. Instruments called Kibble balances measure mass by using precision electrical experiments that rely on this fixed value, linking the unit of mass to a fundamental constant of nature.
The reduced Planck constant equals the Planck constant divided by 2π and is commonly denoted h-bar. Paul Dirac introduced the h-bar symbol in his 1930 book The Principles of Quantum Mechanics, after the combination had appeared in the literature for about 15 years without a dedicated symbol.