Planck's constant is a thing we all hear about in high school physics classes, but no one really talks about why it's cool.
Textbooks will talk dryly about "blah blah theories of quantum gravity" and teachers will repeat that "something something a photon's energy to its frequency. And sure, these things are true. But when people say stuff like that, they're really burying the lede.
Planck's constant is about the limits of what's physically possible in the Universe!
A Planck length is a unit of length which can be derived from Planck's constant. It is the smallest length which is possible in our Universe.
And it's unimaginably tiny. A hundredth of a billionth of a billionth of the size of a proton. But this is significant. Anything smaller than a Planck length cannot exist. It would not make physical sense.
Planck lengths are basically Universe pixels.
I think it also plays an important role in strings theory as it determines the size of the strings.
@InvaderXan Absolutely - merely noting Planck length is at the base of the theory!
@kravietz Assuming strings actually exist