It is essentially a proportionality constant between the energy of a quantum of radiation and the radiation frequency. It has dimensions of action and angular momentum, all of which is very important throughout any quantum theory, needless to say.
"He adduced additional evidence for the importance of his formula by using it to evaluate the constant h (his value was 6.55 ´ 10-27 erg-second, close to the modern value), as well as the so-called Boltzmann constant (the fundamental constant in kinetic theory and statistical mechanics), Avogadro's number, and the charge of the electron."
is not very helpful either, eh? I think he actually made a guess of it at first, with help of calculations to make it ever more accurate. Something quickly buried in this hagiographic 'it all came to me in a divine flash' type of scientific history-writing, we have altogether too much of these days. They are portraying these guys like some demigods, not your average dead-ordinary intelligent or even brilliant bloke full of human doubts, and at times woefully stuck, that they simply were at times.
'People we can never hope to emulate.' Hah. Hagiography, I say. Mythbuilding. Pfui.http://uk.geocities.com/max_planck_uk/people/planck/planck.html
The living are the dead on holiday. -- Maurice de Maeterlinck (1862-1949)