what does it mean?

this often happens when a horizontal plain folds and outcroppings appear.

This is my understanding; I am happy to be corrected by a geologist. A “horizontal plain” is a large, flat area of land. This may be “folded” (imagine a tablecloth being crumpled) by geological processes, creating humps and bumps. As the humps and bumps erode, parts of the underlying rocks are left exposed as “outcroppings” .

Thank you very much dear Dozy