But what is a quantum, and what does it have to do with quantum mechanics? A quantum is any quality that must occur in discrete, non-continuous increments. Consider the two rows of color shown above. In the top one, light blue transitions to dark blue continuously, moving through all the in-between values on the way. If you characterized the shades of blue with numbers, you might say the color of that bar could have a blue of 1, a blue of 1.001, a blue of 1.002, all the way up to a blue of 5.
On the bottom, however, the blue is quantized, occurring in only 5 values: 1, 2, 3, 4, or 5. That’s it. There can be no in-between values. What Planck discovered about light was that its energy was like the blue in the bottom row: it could only have distinct values, and going from one to the other meant adding another photon. (Photons emitted by the metal in the photo are making it glow.)
So what does this have to do with mechanics? Perhaps light is not the only thing that has quantized energy. In fact this is the case: the energy levels of the particles that make up atoms, and therefore everything, also have discrete quanta.