Here's a breakdown:

* Compressional Stress: This happens when forces push against each other from opposite directions. Imagine squeezing a piece of clay – the clay will buckle and fold.

* Anticlines: These are upward folds, resembling an arch. The oldest rocks are in the core of the anticline.

* Synclines: These are downward folds, shaped like a trough. The youngest rocks are in the core of the syncline.

In essence, the compressional stress causes the Earth's crust to buckle and bend, resulting in the formation of anticlines and synclines.