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, is in simplest form, since 5 and 9 have no common factors other than 1. But is not; 6 and 9 have a common factor 3.
To write in simplest form, divide both the numerator and denominator by the greatest common factor, in this case 3:
So in simplest form is .
This is known as reducing fractions.