Isotopes of an element are its different forms in which it has different atomic masses. The number of protons are equal in all forms but the number of neutrons are different.
For example, hydrogen has isotopes
Protium - one proton, no neutron (Atomic mass = 1)
Deuterium - one proton, 1 neutron (Atomic mass = 2)
Tritium - one proton, 2 neutrons (Atomic mass = 3)