All matter is made up of atoms. The protons and neutrons combine to create the nucleus, which is the centre of an atom. However, the elements atomic mass is equal to the number of protons plus the number of neutrons. All atoms of the same element have the same number of protons, but they do not have to have the same number of neutrons. In other words, atoms of the same element can have a different atomic mass.
Most elements have several isotopes.
Isotopes have the same chemical properties, but often have slightly different physical properties, due to the differences in their mass. Many naturally occurring isotopes are stable, which means that their nuclei remain the same and do not change spontaneously. However, in some cases, a particular isotope, or several isotopes even, of an element are unstable. Unstable isotopes want to become stable. In some cases, a particular unstable isotope will decay into another unstable isotope, which will itself then decay, and so on, until a stable isotope is finally reached.