Metamorphic facies are groups of metamorphic rocks that share similar mineral compositions and textures, formed under specific temperature and pressure conditions. They help geologists understand the metamorphic history of a region by indicating the environment in which the rocks were altered. Each facies is characterized by particular minerals that are stable at certain conditions. For example, the greenschist facies forms at lower temperatures and pressures, while the granulite facies develops under higher temperatures and pressures. By studying these facies, scientists can infer the tectonic processes and geological history of an area.