Can someone explain Planck's constant simply?

Others will probably answer much more complex versions, and they will explain much better. But here is a very simple answer:

Its numerical value is the energy of a single photon with the frequency of $1 Hz$.

From another point of view, you can see $h$ as a spatial and temporal scale factor where quantum wave effects become more or less relevant. Changing h is like "zooming in/out"; if you reduce $h$, it's like zooming out, at which point quantum waves oscillate in such a small scale that their interference effects average out into a more classical world.

This way of seeing it can be useful for intuition.