Metamorphic rocks are called this way because they are formed when other rocks "morph" into a different rock.
Due to the movement of the crust, rocks are sometimes pushed underground. Between 100 and 200 kilometers under the earth, rocks start melting from the heat of the core. When this happens, some rocks mix while melting, thus forming metamorphic rocks.
Another factor that can morph rocks together is pressure. Heat and pressure usually work together to morph two rocks together. Heat melts the rocks, and pressure pushes them together.