A fraction is said to be in **simplest form **if its numerator and denominator are relatively prime, that is, they have no common factors other than $1$. (Some books use "written in lowest terms" to mean the same thing.)

So, $\frac{5}{9}$ is in simplest form, since $5$ and $9$ have no common factors other than $1$. But $\frac{6}{9}$ is not; $6$ and $9$ have a common factor $3$.

To write $\frac{6}{9}$ in simplest form, divide both the numerator and denominator by the greatest common factor, in this case $3$:

$\frac{6\text{\hspace{0.17em}}\xf7\text{\hspace{0.17em}}3}{9\text{\hspace{0.17em}}\xf7\text{\hspace{0.17em}}3}=\frac{2}{3}$

So $\frac{6}{9}$ in simplest form is $\frac{2}{3}$.

This is known as reducing fractions.