Metamorphic Rocks Form Through Heat and Pressure
Metamorphic rock is the result of the transformation of an existing rock type by a process called metamorphism which literally means 'change in form'. Metamorphic rocks have been subjected to intense heat and pressure which subsequently causes a profound physical and/or chemical change in their composition. Despite the amount of heat these rocks are exposed to instead of melting they're transformed into a denser more compact type of rock. If the rocks were exposed to enough heat to cause them to melt once the material solidified it would be an igneous rock.
Before being metamorphosed metamorphic rocks would have either been igneous or sedimentary rocks. When the chemical composition of a rock is changed new minerals are created.
Movement in the Earth causes the heat and pressure that's required to create a metamorphic rock. Marble which began life as limestone is a common example of a metamorphic rock.