Here's a breakdown of what that means:

* Metamorphic Rocks: These rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical reactions deep within the Earth.

* Foliation: This refers to the development of planar (flat) structures within a rock. These structures are often visible as parallel bands or layers.

Why does foliation happen?

* Pressure: Intense pressure from tectonic forces can cause minerals in the original rock to align themselves perpendicular to the direction of pressure. This creates a layered appearance.

* Recrystallization: Heat and pressure cause the minerals in the rock to change their size and shape, and they often recrystallize into new minerals that are aligned.

Examples of foliated metamorphic rocks:

* Slate: A fine-grained, low-grade metamorphic rock with a well-developed foliation, often found in roofing tiles.

* Phyllite: A slightly higher grade metamorphic rock than slate, with a silky sheen due to the alignment of mica minerals.

* Schist: A medium-grade metamorphic rock with larger, easily visible mineral grains that have aligned to form a distinct foliation.

* Gneiss: A high-grade metamorphic rock with alternating bands of light and dark minerals. The bands can be thick and wavy, giving the rock a distinctive appearance.

Non-foliated metamorphic rocks:

There are also metamorphic rocks that do not have a layered structure. These are called non-foliated metamorphic rocks. They are typically formed under conditions where pressure is more evenly distributed. Examples include marble (formed from limestone) and quartzite (formed from sandstone).