The atomic mass unit (amu) is a unit of mass used to express the relative masses of atoms and molecules. The amu is defined as one-twelfth the mass of an atom of carbon-12, which is assigned a mass of exactly 12 amu.
Therefore, an atom of carbon-12 has a mass of 12 amu. In comparison, an atom of hydrogen-1 (the most common isotope of hydrogen) has a mass of approximately 1.0078 amu. So, an atom of hydrogen-1 is often rounded to having a mass of 1 amu for simplicity, even though it is slightly heavier.