A lot of video explanations spend the first ten minutes explaining the words 'chromatic' and 'mediant'. They are good words to understand, but I'll skip that and get to the point.

Say you play a minor triad - for example, the notes in red here. Then, starting on the second of those red notes, you play another minor triad - the notes in blue. This gives you a chromatic mediant!

This has a creepy effect. Why?

Both chords have the same shape, and they share one note, so the move isn't too jarring. But the second chord is in a different key than the first. This gives the sequence a floating quality, as if you're in a balloon and someone cut the rope and you start drifting upward. And the note in the new key that's not in the original key is the tritone of the first key! This massively boosts the creepiness.

You can also do chromatic mediants in a major key, which have a different feel.

(2/n)