Before Planck, it was thought that energy, frequency, all of those measurements were a smooth continuous spectrum. You could always add another decimal. You could emit something at 99.99999 hertz and also at 99.9999999999 hertz, etc.

Planck realized there's a problem here. He was looking at something called black body radiation, which is basically an object that emits radiation at all frequencies. But if you allow frequencies to be defined infinitely close to one another, and it emits at "all" frequencies, doesn't that mean it emits an infinite amount of energy? After all, you could always define another frequency .00000000000000000001 between the last two you defined and say it emits at that too.

Obviously this doesn't happen. So Planck theorized that there is a minimum "resolution" to frequencies and energy. Through both experimentation and theory, he realized that all the frequencies and energies radiated were multiples of a single number, which came to be called Planck's constant. To simplify, you could emit at say, 10000 Planck's constants, and at 10001, but not at 10000.5.

Because energy, frequency, mass, matter, etc. are all related through other theories, this minimum "resolution" to energy has enormous implications to everything in physics. It's basically the minimum resolution to the whole universe.

Because nothing travels faster than light, and mass and space and time and the speed of light are related, you can derive things from it like Planck Time (the smallest possible measurable time), Planck Length (the smallest possible measurable distance), etc. In a way, it's basically the constant that defines the size of a "pixel" of reality.

(Edit: a number of people have called out that the quantization does not happen at the frequency level. This is correct, but given the constant's proportional relationship between the discrete energy level of an oscillator vs. the frequency E=hf I figured I could skip over this and treat the frequency as discrete in the answer and move on. Remember most of the audience doesn't even know what a photon is. The tradeoffs over oversimplification for ELI5.)