An isotope refers to atoms of an element which differ only in the number of neutrons they have. An element is uniquely defined by the number of protons it has. For example an atom with 6 protons in its nucleus is called a carbon atom. The number of protons is called the atomic number and it is the same for all atoms of a particular element.
In the case of isotopes of an element the number of neutrons in the nucleus is different. For example, the atomic mass of a normal carbon atom is 12 as it has 6 protons and 6 neutrons in its nucleus. The isotope C-13 also has the same number of protons, i.e. 6, but the number of neutrons is not 6 but 7. This makes the atomic mass 6+7 = 13.
As another example of an isotope we could take tin. Tin has 50 protons and therefore an atomic number of 50. But the number of neutrons in an atom of tin is 68 or 70 which makes the atomic mass 118 or 120 resp.
Isotope are the element that has same atomic number but different mass number.
The best example is Uranium(radioactive) there are two isotope of uranium they are U-235 and U-238. Both have same atopmic number but different mass number 238 and 235. same goes for carbon that is Carbon-12 and carbon 14(radioactive)