Proportionality factor in the Planck-Einstein relation between light energy and frequency.
And analogously for matter, appears in the de Broglie relations relating momentum and frequency.
A very direct way of thinking about it is to think about what would happen in a double-slit experiment.
Also appears in the Schrodinger equation, basically as a consequence/cause of the de Broglie relations most likely.
Defined exactly in the 2019 revision of the International System of Units.