What is relative abundance of an isotope?
The relative amount in which each isotope is present in an element is called its isotopic/relative abundance
Isotopes, which I'm guessing you know but I'll explain it here anyway, are the atoms of an element, that are chemically similar, but have different numbers of neutrons, and thus, diff. mass numbers.
So for example, magnesium could be in the following forms
Mg-24, Mg-25, Mg- 26. All have different mass numbers, ONLY DIFFERENT MASS NUMBERS, thats all, their atomic number is the same.
And the mass number of the element which you end up seeing on the periodic table is the average of all the isotopes if youre wondering why their arent all of an elements isotopes present on the periodic table. So yea, the isotopic/relative abundance is just the amount of each isotope present in an element
Sorry if i confuse you, first time answering :P