The number next to isotopes signifies the sum of the number of protons plus the number of neutrons in the nucleus of an atom. For a particular element, the number of protons never changes, but the number of neutrons can vary.
All atoms of the element chlorine, have 17 protons in their nucleus. The isotope chlorine-28 has 11 neutrons in its nucleus, while chlorine-37 has 20 neutrons. The number of protons and neutrons largely determines the atomic mass of an atom, so different isotopes of an element differ in their mass. Uranium-238, which has 92 protons and 146 neutrons, has a higher atomic mass than does uranium-235, which has only 143 neutrons.