Relative mass is the average mass of a large number of atoms of a particular element on the periodic table. It is typically shown on the bottom of each element's box on the periodic table.
The mass of an individual atom is the sum of its protons and neutrons, as electrons have insignificant mass. Elements in chemistry have relative mass because a subset of atoms corresponding to each element are isotopes. These are atoms of the element with the same number of protons but a higher number of neutrons. Relative mass is affected by the abundance of the isotope in each element.