what is  Relative abundance ?

Relative abundance means in what percent the give substance or element is found in earth's atmosphere in relative to some other substance or element.  

for example...

The relative abundance of two isotopes say A and B of copper,  are 69.09% and 30.91% respectively.

that indicates that , if we take a sample of 100g copper, it will contain, 69.09g of isotope A and 30.91 g of isotope B.