Here's a breakdown:

* Shale: A fine-grained sedimentary rock formed from the accumulation of mud and clay.

* Metamorphism: The process of transforming existing rocks (like shale) into new rocks through heat and pressure.

* Slate: A metamorphic rock formed from shale that has been subjected to low-grade metamorphism. This means the heat and pressure were relatively low, but enough to recrystallize the clay minerals in shale into smaller, more tightly packed crystals.

The heat and pressure usually come from tectonic forces like the collision of continental plates.