The three agents of metamorphism are heat, pressure and fluids. Note that only certain fluids can cause metamorphism, typically those with dissolved ions.
All pre-existing rock is known as protoliths. These can be formed into metamorphic rock through any one of the three agents of metamorphism. A metamorphic rock is any rock that becomes altered, usually by becoming denser, having its mineral content changed or by having a different texture formed. Heat often results in certain minerals becoming stable or unstable, resulting in a changed composition. High pressure over time compresses rock into a denser form. Finally, ionized liquids can cause mineral chemical reactions to occur or support changes from the other two agents.