Related denominators occur where one denominator is a multiple of the other; for example, the fractions $$\frac13$$ and $$\frac59$$ have related denominators because 9 is a multiple of 3.
Fractions with related denominators are more easily added and subtracted than fractions with unrelated denominators, because only one needs to be rewritten; for example, to add $$\frac13$$ and $$\frac59$$ we can rewrite $$\frac13$$ as the equivalent fraction $$\frac39$$ and then compute $$\frac39+\frac59$$ = $$\frac89$$.