What is occlusion and ambient shadow?

https://www.instagram.com/p/B3w9LdAjWb1/

This artist in the link above describes her process in the caption. On step 5, it reads: 

5. Add occlusion and ambient shadow (HSB adjustment layer again) and small light details if necessary (just for accents)

This step refers to the last "snapshot" of her piece, when you scroll all the way to the right. If you scroll one step to the previous one, you can see what it looked like before applying step 5. 

In Photoshop, what does it mean to add occlusion and ambient shadow? Are these filters? Or are these general painting techniques? I'm basically painting my own version of this piece following her specific steps, but when I get to step 5, I'm stumped.

Thank you