I have read some books and references about quantum mechanics and [black body radiation](https://en.wikipedia.org/wiki/Black-body_radiation) but still I didn't understand that what is the black body in fact and how did Planck solve the problem of black body? in the chart of the [Planck](https://en.wikipedia.org/wiki/Planck%27s_law) for the black body radiation, we see that after a frequency, our line comes down and energy decreases. but it is not logical. where does this energy go and what happens to that high frequency?