He was focusing on the form, shape, and darkness of the cliff along the waterfall. And when he said that, it brought to light that just because he is putting “big cliffs, rocks, and little happy things,” doesn’t mean you have to. In your world (or painting), you can have as much creative freedom (or cliffs) as you want.
It’s okay to be nervous. Sometimes being nervous is a good thing. Sometimes it’s worth it to take a leap of faith, and just try something that you may not be comfortable with (even if it is adding a rock in a painting).
After he said that, for the remainder of the video I solely focused on that mountain. If he didn’t say that, I may have focused more on the waterfall; but it showed that sometimes things are there just because of how they look and they don’t need to be bothered with.