Nitrogen-14 has a mass number of 14 which means that it has a total of 14 protons and neutrons. When you subtract the atomic number of nitrogen from the mass number of 14 you get the number of neutrons (14-7), which gives you 7 neutrons. Now for nitrogen-15, you have a different mass number which means there is a different amount of neutrons (15-7), 8 neutrons. So the difference is the mass numbers and the number of neutrons (the number of protons remains the same however). When there are two versions of the same element, and they have the same number of protons, but different number of neutrons and different mass numbers, they are called isotopes. Hope this helped :)